diff --git a/cassandra/advanced/collection-data-types.md b/cassandra/advanced/collection-data-types.md
new file mode 100644
index 0000000..cc56070
--- /dev/null
+++ b/cassandra/advanced/collection-data-types.md
@@ -0,0 +1,63 @@
+CQL also provides collection data types. Following list provides different data types available in cassandra.
+
+1. List - Collection of one or more ordered elements.
+1. map - Collection of key-value pairs.
+1. set - Collection of one or more elments.
+
+### List
+List is used to maintain the order of elements allowing duplicates.
+
+* Creating a Table with List
+```sh
+CREATE TABLE employee(id int PRIMARY KEY, name text, email list<text>);
+```
+
+* Inserting Data into List
+```sh
+INSERT INTO employee(id, name, email) VALUES (1, 'john',
+['john@gmail.com','john123ba@yahoo.com'])
+```
+* Updating a List
+```sh
+UPDATE employee SET email = email +['johnabc@live.com']
+WHERE name = 'john';
+```
+
+### SET
+SET is used to store group of data and the elements will be returned in sorted order.
+
+* Creating a Table with Set
+```sh
+CREATE TABLE employee2(id int PRIMARY KEY, name text, mobile set<varint>);
+```
+
+* Inserting Data into Set
+```sh
+INSERT INTO employee2(id, name, mobile) VALUES (1, 'kevin',
+{9999911122, 9998855664})
+```
+* Updating a List
+```sh
+UPDATE employee2 SET mobile = mobile +{9898989898}
+WHERE name = 'kevin';
+```
+
+
+### MAP
+Map is used to store key-value pair of elements.
+
+* Creating a Table with Map
+```sh
+CREATE TABLE employee3(id int PRIMARY KEY, name text, address map<text, text>);
+```
+
+* Inserting Data into Map
+```sh
+INSERT INTO employee3(id, name, address) VALUES (1, 'smith',
+{'home':'san jose','work':'chicago'})
+```
+* Updating a Map
+```sh
+UPDATE employee3 SET address = address +{'permanent':'New york'}
+WHERE name = 'smith';
+```
diff --git a/cassandra/crud-operations/creating-data.md b/cassandra/crud-operations/creating-data.md
new file mode 100644
index 0000000..a2d4ea9
--- /dev/null
+++ b/cassandra/crud-operations/creating-data.md
@@ -0,0 +1,21 @@
+INSERT command is used to insert data into columns of the table.
+
+Syntax
+
+```sh
+INSERT INTO <tablename>  
+(<column1 name>, <column2 name>....)  
+VALUES (<value1>, <value2>....)  
+USING <option>   
+```
+
+
+Example : 
+```sh
+INSERT INTO student (id, name, city)   
+VALUES(1, 'Paul', 'Texas');  
+INSERT INTO student (id, name, city)   
+VALUES(2, 'John', 'New York');
+INSERT INTO student (id, name, city)   
+VALUES(1, 'Max', 'New Jersey');
+```   
diff --git a/cassandra/crud-operations/creating-keyspace.md b/cassandra/crud-operations/creating-keyspace.md
new file mode 100644
index 0000000..e9bad0b
--- /dev/null
+++ b/cassandra/crud-operations/creating-keyspace.md
@@ -0,0 +1,41 @@
+A keyspace is RDBMS database which contains tables, columns and data.
+
+### Syntax
+```sh
+CREATE KEYSPACE <identifier> WITH <properties>   
+```
+
+```sh
+Create keyspace KeyspaceName with replicaton={'class':strategy name,   
+'replication_factor': No of replications on different nodes}   
+```
+
+### Strategy, Replication Factor
+
+There are two types of strategy declarations.
+
+* Simple Strategy - It is used in case of single data center. This is not a wise choice for production, it may lead to latency.
+* NetworkTopologyStrategy - It is used in case of more data centers. Replication factor is provided for each data center separately.
+
+Replication Factor- Replication factor is the number of replicas of data placed on different nodes. Atleast 3 replication factor is required to attain no single point of failure.
+
+Example
+
+```sh
+CREATE KEYSPACE whichdb  
+WITH replication = {'class':'SimpleStrategy', 'replication_factor' : 3};  
+ ```
+
+
+ DESCRIBE Keyword is used to display all the keyspaces created.
+```sh
+DESCRIBE keyspaces
+
+system_schema system system_traces
+system_auth system_distributed whichdb
+ ```
+
+USE keyword is used to use the keyspace
+```sh
+USE whichdb
+```
\ No newline at end of file
diff --git a/cassandra/crud-operations/creating-table.md b/cassandra/crud-operations/creating-table.md
new file mode 100644
index 0000000..86504ec
--- /dev/null
+++ b/cassandra/crud-operations/creating-table.md
@@ -0,0 +1,30 @@
+CREATE command is used to create a table. Here column family is used to store data just like tables in RDBMS.
+
+Syntax
+
+```sh
+CREATE (TABLE | COLUMNFAMILY) <tablename>  
+('<column-definition>' , '<column-definition>')  
+(WITH <option> AND <option>)   
+```
+
+or
+
+```sh
+CREATE TABLE tablename(  
+   column1 name datatype PRIMARYKEY,  
+   column2 name data type,  
+   column3 name data type.  
+   )  
+
+```
+
+Example : 
+```sh
+CREATE TABLE student(  
+   id int PRIMARY KEY,  
+   name text,  
+   city text,     
+   mobile varint  
+);
+```   
diff --git a/cassandra/crud-operations/deleting-data.md b/cassandra/crud-operations/deleting-data.md
new file mode 100644
index 0000000..6b63d9a
--- /dev/null
+++ b/cassandra/crud-operations/deleting-data.md
@@ -0,0 +1,22 @@
+DELETE command is used to delete data from cassandra table, You can delete complete table or particular data based on condition
+
+```sh
+DELETE FROM <identifier> WHERE <condition>;   
+```
+
+### Deleting an Entire Table
+```sh
+DELETE FROM student;   
+```
+
+### Deleting an Particular Data based on condition
+WHERE clause is specified.
+```sh
+DELETE FROM student where id = 3;   
+```
+
+### Deleting an specific column name
+```sh
+DELETE city FROM student WHERE id=4;   
+```
+
diff --git a/cassandra/crud-operations/deleting-keyspace.md b/cassandra/crud-operations/deleting-keyspace.md
new file mode 100644
index 0000000..70af373
--- /dev/null
+++ b/cassandra/crud-operations/deleting-keyspace.md
@@ -0,0 +1,5 @@
+Drop command is used to drop keyspaces with all the data, columns and indexes from cassandra
+
+```sh
+DROP keyspace whichdb
+```
\ No newline at end of file
diff --git a/cassandra/crud-operations/reading-data.md b/cassandra/crud-operations/reading-data.md
new file mode 100644
index 0000000..6151c0c
--- /dev/null
+++ b/cassandra/crud-operations/reading-data.md
@@ -0,0 +1,23 @@
+## Reading whole table from a keyspace
+SELECT is used to read data from cassandra table.
+
+```sh
+SELECT FROM <tablename>   
+
+SELECT * FROM student;  
+```
+
+## Reading Particular columns in a table
+Required columns are specified instead of *.
+```sh
+SELECT id, name, city FROM student;   
+```
+
+## Reading data from table based on condition
+WHERE clause is used to return a data based on given condition
+
+```sh
+SELECT * FROM student WHERE id=2;    
+```
+
+
diff --git a/cassandra/crud-operations/updating-data.md b/cassandra/crud-operations/updating-data.md
new file mode 100644
index 0000000..a21bd91
--- /dev/null
+++ b/cassandra/crud-operations/updating-data.md
@@ -0,0 +1,27 @@
+UPDATE command is used to update the data in table. Where clause can used to update rows based on condition.
+
+
+Syntax
+```sh
+UPDATE <TABLE>  
+SET <COLUMN> = <VALUE>  
+<COLUMN> = <value>....  
+WHERE <condition>   
+```
+
+## Update multiple values
+```sh
+UPDATE <TABLE>  
+SET <COLUMN1> = <VALUE1>,  
+SET <COLUMN2> = <VALUE2>  
+<COLUMN> = <value>....  
+WHERE <condition>   
+```
+
+
+### Example
+
+```sh
+UPDATE student SET city='New York',name='Rahul'  
+WHERE id=2;   
+```
\ No newline at end of file
diff --git a/cassandra/index.json b/cassandra/index.json
new file mode 100644
index 0000000..99f2cb7
--- /dev/null
+++ b/cassandra/index.json
@@ -0,0 +1,60 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title": "What is Cassandra",
+                "path": "what-is-cassandra"
+            },
+            {
+                "title": "Key Concepts",
+                "path": "key-concepts"
+            },
+            {
+                "title": "Installation",
+                "path": "installation"
+            }
+        ]
+    },
+    {
+        "title": "CRUD Operations",
+        "path": "crud-operations",
+        "links": [
+            {
+                "title": "Creating Keyspace",
+                "path": "creating-keyspace"
+            },
+            {
+                "title": "Deleting Keyspace",
+                "path": "deleting-keyspace"
+            },
+            {
+                "title": "Creating Table",
+                "path": "creating-table"
+            },
+            {
+                "title": "Inserting Data",
+                "path": "creating-data"
+            },
+            {
+                "title": "Updating Data",
+                "path": "updating-data"
+            },
+            {
+                "title": "Deleting Data",
+                "path": "deleting-data"
+            }
+        ]
+    },
+    {
+        "title": "Advanced Topics",
+        "path": "advanced",
+        "links": [
+            {
+                "title": "Collection Data types",
+                "path": "collection-data-types"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/cassandra/introduction/installation.md b/cassandra/introduction/installation.md
new file mode 100644
index 0000000..2340e5e
--- /dev/null
+++ b/cassandra/introduction/installation.md
@@ -0,0 +1,32 @@
+## Installing on MacOS
+
+Easiest way to get Cassandra installed on MacOS is via Homebrew. You can run the following command to install Cassandra in MacOS
+```sh
+brew install cassandra
+```
+
+## Installing on Windows
+
+We can download the latest version of cassandra from the following website
+http://cassandra.apache.org/download/
+
+Run the installation and open cassandra
+
+### Installing Fron Debian Packages
+```sh
+echo "deb http://www.apache.org/dist/cassandra/debian 311x main" | sudo tee -a /etc/apt/sources.list.d/cassandra.sources.list
+sudo apt-get update
+sudo apt-get install cassandra
+```
+
+### Installing Fron RPM Packages
+```sh
+[cassandra]
+name=Apache Cassandra
+baseurl=https://www.apache.org/dist/cassandra/redhat/311x/
+gpgcheck=1
+repo_gpgcheck=1
+gpgkey=https://www.apache.org/dist/cassandra/KEYS
+
+sudo yum install cassandra
+```
\ No newline at end of file
diff --git a/cassandra/introduction/key-concepts.md b/cassandra/introduction/key-concepts.md
new file mode 100644
index 0000000..576c87d
--- /dev/null
+++ b/cassandra/introduction/key-concepts.md
@@ -0,0 +1,17 @@
+## Cassandra Query Language
+Cassandra Query Language (CQL) is used to access Cassandra database through queries. CQL Treats Database as keyspaces as a container of tables.
+
+## Terms Related to Cassandra
+* Node  - Node is place where data is stored.
+* Data Center - Data center is a collection of related nodes.
+* Cluster - Cluster is component containing one or more data centers.
+* Commit log - This is a crash recovery mechanism, every write operation is written into the log.
+
+## Data Replication in Cassandra
+In cassandra nodes in a cluster acts as replicas for a given piece of data. If a node responds with out-of-date value, Cassandra will return the recent value and performs a repair in the background to repair the old value.
+
+## Keyspace, Table, Data
+Cassandra provides keyspaces(Database) which contains multiple tables. Tables contain data
+
+## Cassandra CQLsh
+Cassandra CQL shell is utility to interact with Cassandra using CQL Queries. 
diff --git a/cassandra/introduction/what-is-cassandra.md b/cassandra/introduction/what-is-cassandra.md
new file mode 100644
index 0000000..5931dff
--- /dev/null
+++ b/cassandra/introduction/what-is-cassandra.md
@@ -0,0 +1 @@
+Cassandra is a free and open-source, distributed NoSQL database management system. It is provided by Apache. It is designed to handle huge amount of data providing high performance, high scalability across servers without any failure.
\ No newline at end of file
diff --git a/couchdb/crud-operations/creating-data.md b/couchdb/crud-operations/creating-data.md
new file mode 100644
index 0000000..4cd91cc
--- /dev/null
+++ b/couchdb/crud-operations/creating-data.md
@@ -0,0 +1,39 @@
+CouchDB curl utility provides PUT method to insert data/document into couchdb database.
+
+Syntax : 
+```sh
+curl -X PUT http://127.0.0.1:5984/<database>/"id" -d ' { document } '   
+```
+Example :
+```sh
+curl -X PUT \
+  http://127.0.0.1:5984/employee/1 \
+  -H 'Content-Type: application/json' \
+  -d '{
+    "name": "John",
+    "city": "New york",
+    "mobile": 9921829381
+}'
+
+{
+    "ok": true,
+    "id": "1",
+    "rev": "1-fef96a337fdc6197293y41fs64a4d2ea"
+}
+```
+Document id, datbase name needs to be specified in curl request.
+
+Document can be displayed in the following manner.
+
+```sh
+curl -X GET http://127.0.0.1:5984/employee/1
+
+{
+    "_id": "1",
+    "rev": "1-fef96a337fdc6197293y41fs64a4d2ea",
+    "name": "John",
+    "city": "New york",
+    "mobile": 9921829381
+}
+```
+
diff --git a/couchdb/crud-operations/creating-database.md b/couchdb/crud-operations/creating-database.md
new file mode 100644
index 0000000..c482d71
--- /dev/null
+++ b/couchdb/crud-operations/creating-database.md
@@ -0,0 +1,26 @@
+A database is outermost structure which contains documents. You can create a database using curl in the following way.
+
+```sh
+curl -X PUT http://127.0.0.1:5984/employee  
+```
+
+If the database is succesfully created, you will receive following response
+```sh
+{
+   "ok": true
+}
+```
+
+If you try to create database with same name, you will receive following response
+```sh
+{
+    "error": "file_exists",
+    "reason": "The database could not be created, the file already exists."
+}
+```
+
+You can display all the databases created in the followin manner
+```sh
+curl -X GET http://127.0.0.1:5984/_all_dbs
+["employee"]
+```
\ No newline at end of file
diff --git a/couchdb/crud-operations/deleting-data.md b/couchdb/crud-operations/deleting-data.md
new file mode 100644
index 0000000..ae6ccac
--- /dev/null
+++ b/couchdb/crud-operations/deleting-data.md
@@ -0,0 +1,40 @@
+CouchDB curl utility provides DELETE method to delete data/document in couchdb database.
+
+Syntax : 
+```sh
+curl -H 'Content-Type: application/json' \  
+-X DELETE http://127.0.0.1:5984/<document>/<doc_id>?rev=2-5fef7ea4661b53c017e167809e4f2beb  
+```
+First you need to retrieve the document using document_id to get _rev of the document.
+```sh
+curl -X GET http://127.0.0.1:5984/employee/1
+
+{
+    "_id": "1",
+    "_rev": "2-fef96a337fdc619lsdfk12owl64a4d2ea",
+    "name": "John",
+    "city": "California",
+    "mobile": 9921829381
+}
+```
+Use the _rev information from above response to delete document in the database
+
+Example :
+```sh
+curl -X DELETE \
+  'http://127.0.0.1:5984/employee/1?rev=2-fef96a337fdc619lsdfk12owl64a4d2ea' \
+
+{
+    "ok": true,
+    "id": "1",
+    "rev": "3-fef96a337f2dab20d11b572ow4a4d2ea"
+}
+
+curl -X GET http://127.0.0.1:5984/employee/1
+
+{
+    "error": "not_found",
+    "reason": "deleted"
+}
+```
+* _rev is required as part of request.
diff --git a/couchdb/crud-operations/deleting-database.md b/couchdb/crud-operations/deleting-database.md
new file mode 100644
index 0000000..087593d
--- /dev/null
+++ b/couchdb/crud-operations/deleting-database.md
@@ -0,0 +1,6 @@
+CouchDB curl utility provided DELETE method to delete the database.
+
+```sh
+curl -X DELETE http://127.0.0.1:5984/employee
+{"ok":true}
+```
\ No newline at end of file
diff --git a/couchdb/crud-operations/updating-data.md b/couchdb/crud-operations/updating-data.md
new file mode 100644
index 0000000..7a1b4a9
--- /dev/null
+++ b/couchdb/crud-operations/updating-data.md
@@ -0,0 +1,40 @@
+CouchDB curl utility provides PUT method to update data/document in couchdb database.
+
+Syntax : 
+```sh
+curl -X PUT http://127.0.0.1:5984/<database>/<document_id>/ -d '{ "field" : "value", "_rev" : "revision id" }'  
+```
+First you need to retrieve the document using document_id to get _rev of the document.
+```sh
+curl -X GET http://127.0.0.1:5984/employee/1
+
+{
+    "_id": "1",
+    "_rev": "1-fef96a337k65197245f984558a4d2ea",
+    "name": "John",
+    "city": "New york",
+    "mobile": 9921829381
+}
+```
+Use the _rev information from above response to update data in the document
+
+Example :
+```sh
+curl -X PUT \
+  http://127.0.0.1:5984/employee/1 \
+  -H 'Content-Type: application/json' \
+  -d '{
+    "_rev": "1-fef96a337k65197245f984558a4d2ea",
+    "name": "John",
+    "city": "California",
+    "mobile": 9921829381
+}'
+
+{
+    "ok": true,
+    "id": "1",
+    "rev": "1-fef96a337fdc619lsdfk12owl64a4d2ea"
+}
+```
+* _rev is required as part of request.
+* You can only write entirely new version of document i.e you need to specify all the fields even though you want to add single field.
\ No newline at end of file
diff --git a/couchdb/index.json b/couchdb/index.json
new file mode 100644
index 0000000..792985d
--- /dev/null
+++ b/couchdb/index.json
@@ -0,0 +1,46 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title": "What is CouchDb",
+                "path": "what-is-couchdb"
+            },
+            {
+                "title": "Key Concepts",
+                "path": "key-concepts"
+            },
+            {
+                "title": "Installation",
+                "path": "installation"
+            }
+        ]
+    },
+    {
+        "title": "CRUD Operations",
+        "path": "crud-operations",
+        "links": [
+            {
+                "title": "Creating database",
+                "path": "creating-database"
+            },
+            {
+                "title": "Deleting database",
+                "path": "deleting-database"
+            },
+            {
+                "title": "Creating Data",
+                "path": "creating-data"
+            },
+            {
+                "title": "Updating Data",
+                "path": "updating-data"
+            },
+            {
+                "title": "Deleting Data",
+                "path": "deleting-data"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/couchdb/introduction/installation.md b/couchdb/introduction/installation.md
new file mode 100644
index 0000000..0988633
--- /dev/null
+++ b/couchdb/introduction/installation.md
@@ -0,0 +1,7 @@
+To Install CouchDB on your system, Go to official website https://couchdb.apache.org/
+
+You can download required version based on your operating system. It is available for Windows, MacOs, Linux (Ubuntu/CentOS)
+
+CouchDB provides web based built-in administrative interface known as CouchDB Fauxton.
+
+You can access CouchDB Fauxton using http://127.0.0.1:5984/_utils/ .
\ No newline at end of file
diff --git a/couchdb/introduction/key-concepts.md b/couchdb/introduction/key-concepts.md
new file mode 100644
index 0000000..87a4409
--- /dev/null
+++ b/couchdb/introduction/key-concepts.md
@@ -0,0 +1,28 @@
+## Document Oriented
+Document oriented databases are different from traditional RDBMS. 
+Document oriented databases contain documents these are like rows in RDBMS which stores the data. These documents can be complex and have multilevel data. For example, a user document can have a list of roles which is an array of Strings and also can contain address which is another object with its own fields like the dollowing example 
+
+```json
+{
+    "id": 1,
+    "name": "Tom",
+    "roles": ["admin", "manager"],
+    "address" : {
+        "street": "xxxx",
+        "zipcode": 12345,
+        "country": "US"
+    }
+}
+```
+## Replication
+CouchDB provides the simplest form of replication.
+
+## Authentication
+CouchDB can make authentication open through a session cookie like web application.
+
+## CouchDB Fauxton
+CouchDB Fauxton is built-in web based administrative interface to access database. CRUD operations can be easily performed through this system.
+
+## CouchDB Curl
+CouchDB Curl is another utility to interact with CouchDB database. This tool is used to transfer data from or to server by using one of supported protocols(HTTP, HTTPS, FTP). In this tutorial we will be using CouchDB curl using HTTP protocol to explain different operations/commands in CouchDB.
+
diff --git a/couchdb/introduction/what-is-couchdb.md b/couchdb/introduction/what-is-couchdb.md
new file mode 100644
index 0000000..70a1817
--- /dev/null
+++ b/couchdb/introduction/what-is-couchdb.md
@@ -0,0 +1 @@
+Couchdb is a free and open-source, NoSQL database management system. It is implemented in concurrency-oriented language Erlang. It was developed by Apache foundation. 
\ No newline at end of file
diff --git a/dynamodb/index.json b/dynamodb/index.json
new file mode 100644
index 0000000..0634ab9
--- /dev/null
+++ b/dynamodb/index.json
@@ -0,0 +1,60 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title": "What is DynamoDB",
+                "path": "what-is-dynamodb"
+            },
+            {
+                "title": "Key Concepts",
+                "path": "key-concepts"
+            },
+            {
+                "title": "Installation",
+                "path": "installation"
+            }
+        ]
+    },
+    {
+        "title": "CRUD Operations",
+        "path": "crud-operations",
+        "links": [
+            {
+                "title": "Creating Table",
+                "path": "creating-table"
+            },
+            {
+                "title": "Inserting Data",
+                "path": "inserting-data"
+            },
+            {
+                "title": "Reading Data",
+                "path": "reading-data"
+            },
+            {
+                "title": "Updating Data",
+                "path": "updating-data"
+            },
+            {
+                "title": "Deleting Data",
+                "path": "deleting-data"
+            }
+        ]
+    },
+    {
+        "title": "Advanced Topics",
+        "path": "advanced",
+        "links": [
+            {
+                "title": "Secondary Indexes",
+                "path": "secondary-indexes"
+            },
+            {
+                "title": "DynamoDB Streams",
+                "path": "dynamodb-streams"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/dynamodb/introduction/installation.md b/dynamodb/introduction/installation.md
new file mode 100644
index 0000000..e69de29
diff --git a/dynamodb/introduction/key-concepts.md b/dynamodb/introduction/key-concepts.md
new file mode 100644
index 0000000..bb3cc87
--- /dev/null
+++ b/dynamodb/introduction/key-concepts.md
@@ -0,0 +1,24 @@
+## DynamoDB Data Types
+* Scalar  - 
+* Multi-valued - 
+* Document - 
+
+## DynamoDB Data Model
+* Tables  - 
+* items - 
+* Attributes - 
+
+## DynamoDB Primary Keys
+* Simple Primary Key - 
+* Composite Primary Key - 
+
+## DynamoDB Indexes
+* Local Secondary Index (LSI) - 
+* Global Secondary Index (GSI) -  
+
+## DynamoDB Key Features
+* Highly Predicatable Performance - Quality of the service can be controlled by choosing between Strong Consistency (Read-after-Write) or Eventual Consistency and also user can increase or decrease the Read/Write throughput of the service through API calls
+* Provisioned Capacity - 
+* Scalability - As volume of the data grows, it follows automatic partitioning model  and spreads the data across partitions without intervention from user
+
+
diff --git a/dynamodb/introduction/what-is-dynamodb.md b/dynamodb/introduction/what-is-dynamodb.md
new file mode 100644
index 0000000..3669475
--- /dev/null
+++ b/dynamodb/introduction/what-is-dynamodb.md
@@ -0,0 +1 @@
+DynamoDB is a fully managed NoSQL database service having both key-value and document-store. Amazon Web Services(AWS) offers DynamoDB as a hosted database service and runs the infrastructure for you. DynamoDB automatically manages the traffic of tables over multiple servers to handle throughput and storage requirements. It provides high availability, data durability, and fast performance.
\ No newline at end of file
diff --git a/elastic-search/aggregations/bucket-aggregations.md b/elastic-search/aggregations/bucket-aggregations.md
new file mode 100644
index 0000000..9c725d6
--- /dev/null
+++ b/elastic-search/aggregations/bucket-aggregations.md
@@ -0,0 +1,9 @@
+## Date Range Aggregation
+
+## Date Histogram Aggregation
+
+## Filter Aggregation
+
+## Composite Aggregation
+
+## Terms Aggregation
\ No newline at end of file
diff --git a/elastic-search/aggregations/metrics-aggregation.md b/elastic-search/aggregations/metrics-aggregation.md
new file mode 100644
index 0000000..55300b6
--- /dev/null
+++ b/elastic-search/aggregations/metrics-aggregation.md
@@ -0,0 +1,54 @@
+## Avg Aggregation
+Avg Aggregation is a single-value metrics aggregation which computes the average of all numeric values of a field from aggregated documents
+
+Below request will compute the average rank of all documents in `databases` index:
+```json
+POST /databases/_search?size=0
+{
+    "aggs" : {
+        "avg_rank" : { "avg" : { "field" : "rank" } }
+    }
+}
+```
+
+## Max Aggregation
+Max Aggregation is a single-value metrics aggregation which returns the maximum of all numeric values of a field from aggregated documents
+
+Below request will return the maximum rank of all documents in `databases` index:
+```json
+POST /databases/_search?size=0
+{
+    "aggs" : {
+        "max_rank" : { "max" : { "field" : "rank" } }
+    }
+}
+```
+
+## Min Aggregation
+Min Aggregation is a single-value metrics aggregation which returns the minimum of all numeric values of a field from aggregated documents
+
+Below request will return the minimum rank of all documents in `databases` index:
+```json
+POST /databases/_search?size=0
+{
+    "aggs" : {
+        "min_rank" : { "min" : { "field" : "rank" } }
+    }
+}
+```
+
+## Sum Aggregation
+Sum Aggregation is a single-value metrics aggregation which returns the sum of all numeric values of a field from aggregated documents
+
+Below request will return the sum of all rank fields in `databases` index:
+```json
+POST /databases/_search?size=0
+{
+    "aggs" : {
+        "sum_rank" : { "sum" : { "field" : "rank" } }
+    }
+}
+```
+
+
+
diff --git a/elastic-search/crud-operations/deleting-data.md b/elastic-search/crud-operations/deleting-data.md
new file mode 100644
index 0000000..534ac46
--- /dev/null
+++ b/elastic-search/crud-operations/deleting-data.md
@@ -0,0 +1,55 @@
+## Delete API
+This API will delete the document from the `databases` index with ID 1:
+```json
+DELETE /databases/_doc/1
+```
+The result of above request:
+```json
+{
+    "_shards" : {
+        "total" : 2,
+        "failed" : 0,
+        "successful" : 2
+    },
+    "_index" : "databases",
+    "_type" : "_doc",
+    "_id" : "1",
+    "_version" : 2,
+    "_primary_term": 1,
+    "_seq_no": 5,
+    "result": "deleted"
+}
+```
+
+## Delete by Query API
+This `delete_by_query` API deletes every document that matches with given query 
+```json
+POST databases/_delete_by_query
+{
+  "query": { 
+    "match": {
+      "name": "Elastic Search"
+    }
+  }
+}
+```
+The result of above request:
+```json
+{
+  "took" : 14,
+  "timed_out": false,
+  "deleted": 2,
+  "batches": 1,
+  "version_conflicts": 0,
+  "noops": 0,
+  "retries": {
+    "bulk": 0,
+    "search": 0
+  },
+  "throttled_millis": 0,
+  "requests_per_second": -1.0,
+  "throttled_until_millis": 0,
+  "total": 2,
+  "failures" : [ ]
+}
+```
\ No newline at end of file
diff --git a/elastic-search/crud-operations/inserting-data.md b/elastic-search/crud-operations/inserting-data.md
new file mode 100644
index 0000000..e1187e2
--- /dev/null
+++ b/elastic-search/crud-operations/inserting-data.md
@@ -0,0 +1,74 @@
+## Create Document with ID
+This index API adds or updates the JSON document into the `databases` index with an id of 1 :
+```json
+PUT databases/_doc/1
+{
+    "name" : "Elastic Search",
+    "type" : "Document-Oriented Database"
+}
+```
+The result of above request :
+```json
+{
+    "_shards" : {
+        "total" : 2,
+        "failed" : 0,
+        "successful" : 2
+    },
+    "_index" : "databases",
+    "_type" : "_doc",
+    "_id" : "1",
+    "_version" : 1,
+    "_seq_no" : 0,
+    "_primary_term" : 1,
+    "result" : "created"
+}
+```
+## Create Document with Operation Type
+This index API used to force `create` operation on document,which will fail if a document by that id already exists in the index
+
+>Using `op_type` parameter
+```json
+PUT databases/_doc/1?op_type=create
+{
+    "name" : "Elastic Search",
+    "type" : "Document-Oriented Database"
+}
+```
+>Without `op_type` parameter
+```json
+PUT databases/_create/1
+{
+    "name" : "Elastic Search",
+    "type" : "Document-Oriented Database"
+}
+```
+## Create Document with Automatic ID Generation
+This index API adds the JSON document into the `databases` index with an autmatically generated id and also follow `create` operation as default
+```json
+POST databases/_doc/
+{
+    "name" : "Elastic Search",
+    "type" : "Document-Oriented Database"
+}
+```
+The result of above request :
+```json
+{
+    "_shards" : {
+        "total" : 2,
+        "failed" : 0,
+        "successful" : 2
+    },
+    "_index" : "databases",
+    "_type" : "_doc",
+    "_id" : "1WEKA2SMDA0912",
+    "_version" : 1,
+    "_seq_no" : 0,
+    "_primary_term" : 1,
+    "result" : "created"
+}
+```
+
+
+
diff --git a/elastic-search/crud-operations/reading-data.md b/elastic-search/crud-operations/reading-data.md
new file mode 100644
index 0000000..e21570b
--- /dev/null
+++ b/elastic-search/crud-operations/reading-data.md
@@ -0,0 +1,22 @@
+## Get API
+The Get API is used to get JSON document from `databases` index with id value 1:
+```json
+GET databases/_doc/1
+```
+The result of above operation:
+```json
+{
+    "_index" : "databases",
+    "_type" : "_doc",
+    "_id" : "1",
+    "_version" : 1,
+    "_seq_no" : 10,
+    "_primary_term" : 1,
+    "found": true,
+    "_source" : {
+       "name" : "Elastic Search",
+       "type" : "Document-Oriented Database"
+    }
+}
+```
+
diff --git a/elastic-search/crud-operations/search-database.md b/elastic-search/crud-operations/search-database.md
new file mode 100644
index 0000000..cc5c67f
--- /dev/null
+++ b/elastic-search/crud-operations/search-database.md
@@ -0,0 +1,20 @@
+## Search API with query parameter
+This search API is used to execute a search query on the index given and get back results that match the query
+
+Below request will search for Elastic search name in `databases` index:
+```json
+GET /databases/_search?q=name:Elastic search
+```
+
+## Search API with request body
+This search API is used to execute a Query DSL on the index given and get back results that match the query
+
+Below request will search for Elastic search name in `databases` index:
+```json
+GET /databases/_search
+{
+    "query" : {
+        "term" : { "name" : "Elastic search" }
+    }
+}
+```
\ No newline at end of file
diff --git a/elastic-search/crud-operations/updating-data.md b/elastic-search/crud-operations/updating-data.md
new file mode 100644
index 0000000..d5175b7
--- /dev/null
+++ b/elastic-search/crud-operations/updating-data.md
@@ -0,0 +1,36 @@
+## Update API with script
+This update API used to update a document based on a script provided
+
+Below update request adds new field to document: 
+```json
+POST databases/_update/1
+{
+    "script" : "ctx._source.rank = 4"
+}
+```
+Below update request will increment the value of rank field with the value mentioned in `params` :
+```json
+POST databases/_update/1
+{
+    "script" : {
+        "source": "ctx._source.rank += params.count",
+        "lang": "painless",
+        "params" : {
+            "count" : 2
+        }
+    }
+}
+```
+
+## Update API with doc
+This update API is used to pass partial document and merge it into existing document
+
+Below update request adds a new field to existing document:
+```json
+POST databases/_update/1
+{
+    "doc" : {
+        "written_in" : "java"
+    }
+}
+```
diff --git a/elastic-search/index.json b/elastic-search/index.json
new file mode 100644
index 0000000..385e22c
--- /dev/null
+++ b/elastic-search/index.json
@@ -0,0 +1,78 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title": "What is ElasticSearch",
+                "path": "what-is-elasticsearch"
+            },
+            {
+                "title": "Key Concepts",
+                "path": "key-concepts"
+            },
+            {
+                "title": "Installation",
+                "path": "installation"
+            }
+        ]
+    },
+    {
+        "title": "CRUD Operations",
+        "path": "crud-operations",
+        "links": [
+            {
+                "title": "Inserting Data",
+                "path": "inserting-data"
+            },
+            {
+                "title": "Reading Data",
+                "path": "reading-data"
+            },
+            {
+                "title": "Updating Data",
+                "path": "updating-data"
+            },
+            {
+                "title": "Deleting Data",
+                "path": "deleting-data"
+            },
+            {
+                "title": "Search Database",
+                "path": "search-database"
+            }
+        ]
+    },
+    {
+        "title": "Aggregations",
+        "path": "aggregations",
+        "links": [
+            {
+                "title": "Metrics Aggregations",
+                "path": "metrics-aggregation"
+            },
+            {
+                "title": "Bucket Aggregations",
+                "path": "bucket-aggregations"
+            }
+        ]
+    },
+    {
+        "title": "Advanced Topics",
+        "path": "advanced-topics",
+        "links": [
+            {
+                "title": "Indexes",
+                "path": "indexes"
+            },
+            {
+                "title": "Replication",
+                "path": "replication"
+            },
+            {
+                "title": "Sharding",
+                "path": "sharding"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/elastic-search/introduction/installation.md b/elastic-search/introduction/installation.md
new file mode 100644
index 0000000..e871d20
--- /dev/null
+++ b/elastic-search/introduction/installation.md
@@ -0,0 +1,43 @@
+## Installing on MacOS
+
+#### 1. Using Homebrew
+ To install recently released default distribution of Elasticsearch
+ ```
+ brew install elastic/tap/elasticsearch-full
+ ```
+#### 2. Using Archive
+Download the Elasticsearch archive with the version of your choice
+```
+curl -L -O https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-7.3.1-darwin-x86_64.tar.gz
+```
+Extract the archive and start the Elasticsearch from bin
+```
+cd elasticsearch-7.3.1/bin
+./elasticsearch
+```
+
+## Installing on Windows
+
+#### 1. Using Archive
+Download the Elasticsearch archive with the version of your choice
+```
+https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-7.3.1-windows-x86_64.zip
+```
+Extract the archive and start the Elasticsearch from bin
+```
+cd elasticsearch-7.3.1/bin
+./elasticsearch.bat
+```
+
+## Installing on Ubuntu
+
+#### 1. Using Archive
+Download the Elasticsearch archive with the version of your choice
+```
+curl -L -O https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-7.3.1-linux-x86_64.tar.gz
+```
+Extract the archive and start the Elasticsearch from bin
+```
+cd elasticsearch-7.3.1/bin
+./elasticsearch
+```
diff --git a/elastic-search/introduction/key-concepts.md b/elastic-search/introduction/key-concepts.md
new file mode 100644
index 0000000..74afda0
--- /dev/null
+++ b/elastic-search/introduction/key-concepts.md
@@ -0,0 +1,8 @@
+## Node
+## Cluster
+## Index
+## Document
+## Shard
+## Replicas
+## Near Realtime (NRT) 
+
diff --git a/elastic-search/introduction/what-is-elasticsearch.md b/elastic-search/introduction/what-is-elasticsearch.md
new file mode 100644
index 0000000..fbc9de1
--- /dev/null
+++ b/elastic-search/introduction/what-is-elasticsearch.md
@@ -0,0 +1,2 @@
+Elasticsearch is an open source,document-oriented distributed database which is designed to store,retreive and manage structured or unstructured data. Elasticsearch is also a search and analytics engine based on Lucene library. 
+In Elasticsearch, data will be stored in JSON document form. Elasticsearch has it's own Query Domain Specific Language, where JSON format is used to specify the query
diff --git a/firebird/commands/commands.md b/firebird/commands/commands.md
new file mode 100644
index 0000000..a0566e7
--- /dev/null
+++ b/firebird/commands/commands.md
@@ -0,0 +1,19 @@
+
+## ISQL commands
+ 
+
+| Command Syntax | Description|
+|-----|-----|
+|`BLOBDUMP <blobid> <file>`|dump BLOB to a file|
+|`BLOBVIEW <blobid>`| view BLOB in text editor|
+|`EDIT     [<filename>]` |edit SQL script file and execute|
+|`EDIT`                  |edit current command buffer and execute|
+|`HELP `                 |display this menu|
+|`INput    <filename>`   |take input from the named SQL file|
+|`OUTput   [<filename>]` |write output to named file|
+|`OUTput              `  |return output to stdout|
+|`SET      <option>  `   |(Use HELP SET for complete list)
+|`SHELL    <command>`    |execute Operating System command in sub-shell|
+|`SHOW     <object> [<name>]` | display system information where <object> = CHECK, COLLATION, DATABASE, DOMAIN, EXCEPTION, FILTER, FUNCTION,GENERATOR, GRANT, INDEX, PROCEDURE, ROLE, SQL DIALECT, SYSTEM, TABLE, TRIGGER, VERSION, USERS, VIEW|
+|`EXIT `                 |exit and commit changes
+|`QUIT`                  |exit and roll back changes|
diff --git a/firebird/constraints/constraints.md b/firebird/constraints/constraints.md
new file mode 100644
index 0000000..d673708
--- /dev/null
+++ b/firebird/constraints/constraints.md
@@ -0,0 +1,21 @@
+
+Constraint is used to define rules on what values are inserted in the table. Constraint plays an important role in ensuring the accuracy and integrity of the data inserted into a table.
+
+Commonly used constraints are as follows:
+## 1. NOT NULL 
+If a column is specified as NOT NULL while defining the table, then the column must not have NULL values while inserting data into the table.
+## 2. UNIQUE
+ If a column is specified as UNIQUE while defining the table, then the column must have different values for all the rows present in the table.
+## 3. PRIMARY KEY 
+Primary key is a column or set of columns which is used to uniquely identify a row in the database table. In the below customer table, You can either choose InsuranceID or National Insurance Number(NIN) as primary keys, but Insurance ID is preferable as NIN can be considered as personal information.
+#### Table 1: 
+    customer(InsuranceID, Name, DOB, NIN, Location)
+#### Table 2: 
+    patient(HospitalID, Name, DOB, InsuranceID)
+
+## 4. FOREIGN KEY 
+Foreign key is a column in one table which references a primary key in other table. In the above two tables, InsuranceID of patient table can be foreign key which can be referenced to InsuranceID in customer Table.
+## 5. CHECK 
+checks for a specific condition while inserting data into the table. 
+## 6. DEFAULT 
+If a value is not specified for a column whose default value is mentioned while creating the table, then the default value is inserted while inserting data.
diff --git a/firebird/data-types/data-types.md b/firebird/data-types/data-types.md
new file mode 100644
index 0000000..4393bdb
--- /dev/null
+++ b/firebird/data-types/data-types.md
@@ -0,0 +1,49 @@
+# String data types
+
+| Data Type |Description| Maximum Size Limit |
+| --------- | ----------|--------------------|
+|CHAR(length)|Fixed length string, length specifies the number of characters to be stored|255|
+|VARCHAR(length)|Variable length string,length specifies the number of characters to be stored|255|
+|text(length)|Fixed length string, length specifies the number of characters to be stored|255|
+|binary(length)|Fixed length string, length specifies the number of characters to be stored|255| 
+| BLOB, TEXT |BLOB is for storing binary data and Text is used to store large string| 65,535(64KB) |
+| TINYTEXT | used to store short strings |255 |
+| MEDIUMTEXT| used to store larger text strings| 16,777,215(16MB) |
+| LONG TEXT |used to store very large data which can't be stored in MEDIUMTEXT |4,294,967,295(4GB) |
+
+
+# Data and time data types
+
+
+| Data type | Format | Range |
+| ---------- | ------ | ----- |
+| DATE | 'YYYY-MM-DD' | '1000-01-01' to '9999-12-31' |
+| DATETIME[(f)] | 'YYYY-MM-DD hh:mm:ss[.fraction]' | '1000-01-01 00:00:00.000000' to '9999-12-31 23:59:59.999999' | 
+| TIMESTAMP[(f)] | 'YYYY-MM-DD hh:mm:ss[.fraction]' | '1970-01-01 00:00:01.000000' UTC to '2038-01-19 03:14:07.999999' UTC | 
+| TIME[(f)] | 'hh:mm:ss[.fraction]' | '-838:59:59.000000' to '838:59:59.000000' | 
+| YEAR[(2/4)] | 'YY'/'YYYY'(default) | Year represented in 2 or 4 digits|
+
+
+Note: f refers to fractional seconds precision
+
+
+
+# Numeric data types
+
+### 1. Integer Family
+
+| Data Type |	Unsigned Min Value | Unsigned Max Value | Signed 	Min Value | Signed	Max Value |	
+| --------- | -------------------- | ------------------ | ----------------- | ----------------- |
+| INT	|0	| 4294967295 |	-2147483648 |	2147483647 |
+| SMALLINT  | 0	| 65535	| -32768 |	32767|
+| MEDIUMINT	| 0	| 16777215 |	-8388608 |	8388607 |
+| BIGINT	| 0	| 18446744073709551615| 	-9223372036854775808 |	9223372036854775807 |
+| TINYINT	| 0	| 255 |	-128 |	127 |
+
+### 2. FLOAT(d)
+where d is number of decimals. If the d value is in between  0-24 then the data type becomes FLOAT() else if the d value is >24 and <53 then the  data type becomes DOUBLE()
+
+### 3. DECIMAL(l,d)
+where l is number of digits displayed before the decimal point and d is number of digits after the decimal point. Default value is DECIMAL(10,2).
+
+
diff --git a/firebird/index.json b/firebird/index.json
new file mode 100644
index 0000000..10c5dd4
--- /dev/null
+++ b/firebird/index.json
@@ -0,0 +1,97 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title" : "About Firebird",
+                "path" : "about-firebird"
+            },
+            {
+                "title" : "Installation on MacOS, Linux and Windows",
+                "path" : "installation"
+
+            }
+        ]
+    },
+    {
+        "title": "Data Types",
+        "path": "data-types",
+        "links": [
+            {
+                "title": "Data Types",
+                 "path": "data-types"
+            }
+        ]
+    },
+    {
+        "title": "Constraints",
+        "path": "constraints",
+        "links": [
+            {
+                "title": "Constraints",
+                "path": "constraints"
+            }
+        ]
+    },
+    {
+        "title": "Commands",
+        "path": "commands",
+        "links": [
+            {
+                "title": "Commands",
+                "path": "commands"
+            }
+        ]
+    },
+    {
+        "title": "Joins",
+        "path": "joins",
+        "links": [
+            {
+                "title": "Joins",
+                "path": "joins"
+            }
+        ]
+    },
+    {
+        "title": "Indexes",
+        "path": "indexes",
+        "links": [
+            {
+                "title": "Indexes",
+                "path": "indexes"
+            }
+        ]
+    },
+    {
+        "title": "Triggers",
+        "path": "triggers",
+        "links": [
+            {
+                "title": "Triggers",
+                "path": "triggers"
+            }
+        ]
+    },
+    {
+        "title": "Views",
+        "path": "views",
+        "links": [
+            {
+                "title": "Views",
+                "path": "views"
+            }
+        ]
+    },
+    {
+        "title": "Stored Procedures",
+        "path": "stored-procedures",
+        "links": [
+            {
+                "title": "Stored Procedures",
+                "path": "stored-procedures"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/firebird/indexes/indexes.md b/firebird/indexes/indexes.md
new file mode 100644
index 0000000..0cf3d76
--- /dev/null
+++ b/firebird/indexes/indexes.md
@@ -0,0 +1,18 @@
+In simpler terms, we can say INDEX is a pointer to some data in a table. INDEXES are very useful in attaining higher performance especially for large databases as it speeds up your search query execution.
+Unindexed tables take more time when compared to Indexed tables. For example, you can consider the synopsis of a book as an example for Indexes. You can easily search for a particular topic by referring it's page number in synopsis.  
+
+
+## 1. CREATE INDEX
+CREATE INDEX is used to create an index. INDEXes can be created on multiple columns(if required) of a table. Search query corresponding to iNDEX column will be much faster than on other columns.
+
+### Syntax:
+```sql
+  CREATE INDEX index_name on table_name(column_name);
+```
+
+## 2. DROP INDEX
+DROP INDEX is used to delete an existing index
+### Syntax
+```sql
+DROP INDEX index_name ON table_name;
+```
\ No newline at end of file
diff --git a/firebird/introduction/about-firebird.md b/firebird/introduction/about-firebird.md
new file mode 100644
index 0000000..1b97ffc
--- /dev/null
+++ b/firebird/introduction/about-firebird.md
@@ -0,0 +1,12 @@
+Firebird is a relational database management system which is open source and completely free.
+
+## Key Features
+
+* Supports Linux, Windows and a variety of UNIX platforms.
+* It is completely free and you can use it for commercial purpose as well.
+* It supports many ANSI SQL standard features.
+* Powerful language support for Triggers and Stored Procedures. 
+* The development of the firebird features depends on voluntary funding by people who uses it.
+* You can write the commands in `isql` tool and the database files are stored with extension .fdb 
+
+
diff --git a/firebird/introduction/installation.md b/firebird/introduction/installation.md
new file mode 100644
index 0000000..6807ace
--- /dev/null
+++ b/firebird/introduction/installation.md
@@ -0,0 +1,56 @@
+# Installation on MacOS
+
+1. Download the default 1.5.1 package
+2. Unzip the contents into a temp directory
+    ```sh
+    $ cd temp
+    ~/temp $ unzip Firebird-CS-1.5.1-MacOS.zip
+    ```
+3. Install the package using below command
+    ```sh
+    ~/temp $ open Firebird-CS-1.5.pkg
+    ```
+4. Add the below lines to .bash_profile file
+    ```sh
+    export FIREBIRD_HOME=/Library/Frameworks/Firebird.framework/Resources
+    export PATH=$PATH:$FIREBIRD_HOME/bin
+    ```
+
+# Installing and Configuring Superclassic on Linux
+
+## How to install
+1. Download the required version from [Firebird Downloads](http://www.firebirdsql.org/en/firebird-2-5/) 
+2. Add ppa to your system.
+    ```she
+    sudo add-apt-repository ppa:mapopa/ppa
+    sudo apt-get update
+    ```
+3. Inspect firebird2.5 related packages
+    ```sh
+    apt-cache search firebird2.5-*
+    ```
+4. Install the super server package by providing SYSDBA package and select service among Super Server, Classic or Super Classic
+    ```sh
+    sudo apt-get install firebird2.5-super
+    ```
+5. or Install classic using below command
+    ```sh
+    sudo apt-get install firebird2.5-classic
+    ```
+6. or Install super-classic using below command
+    ```sh
+    sudo apt-get install firebird2.5-superclassic
+    ```
+7. Configure the package after the installation.
+    ```sh
+    sudo dpkg-reconfigure firebird2.5-super
+    ```
+8. Install examples and dev files
+    ```sh
+    sudo apt-get install firebird2.5-examples firebird2.5-dev
+    ```
+
+# Installation on Windows
+
+* Installation on windows is much easier using exe file. Download the required version from [Firebird Downloads](https://firebirdsql.org/en/firebird-2-5/)
+* Run the executable file as an administrator and the setup will guide you through the installation and any settings like password etc.
\ No newline at end of file
diff --git a/firebird/joins/joins.md b/firebird/joins/joins.md
new file mode 100644
index 0000000..55dd8ec
--- /dev/null
+++ b/firebird/joins/joins.md
@@ -0,0 +1,90 @@
+
+Joins are used along with SELECT statement when ever there is a need to retrieve data from multiple tables.
+
+Commonly used Joins in MariaDB are as follows:
+
+1. INNER JOIN
+2. LEFT JOIN
+3. RIGHT JOIN
+4. CROSS JOIN
+
+Let us consider the below tables for our understanding:
+
+
+TABLE1: 
+
+
+|A|Z|
+|---|---|
+|a|1|
+|b|4|
+|d|5|
+
+
+TABLE2:                    
+
+|A|X|
+|---|---|
+|a|10|
+|e|17|
+|d|30|
+
+## 1. INNER JOIN
+INNER JOIN combines the values from both the tables based on matching criteria.
+
+Below is a basic example of how inner join works:
+```sql
+SELECT * FROM TABLE1 INNER JOIN TABLE2 where TABLE1.A=TABLE2.A;
+```
+
+#### Result:
+
+|A|Z|A|X|
+|--|--|--|--|
+|a|1|a|10|
+|d|5|d|30|
+
+## 2. LEFT JOIN
+LEFT JOIN returns all the values from the Left hand table of the condition and returns only the matching values from the second table. The unmatched row values of right hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 LEFT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|b|4|null|null|
+
+## 3. RIGHT JOIN
+RIGHT JOIN returns all the values from the right hand table of the condition and returns only the matching values from the left hand table. The unmatched row values of left hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 RIGHT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|null|null|e|17|
+
+## 4. CROSS JOIN
+CROSS JOIN matches each row of the first table with every row of the second table.
+```sql
+SELECT A,Z,X from TABLE1 CROSS JOIN TABLE2;
+```
+#### Result:
+
+|A|Z|X|
+|---|---|---|
+|a|1|10|
+|a|1|17|
+|a|1|30|
+|b|4|10|
+|b|4|17|
+|b|4|30|
+|d|5|10|
+|d|5|17|
+|d|5|30|
diff --git a/firebird/stored-procedures/stored-procedures.md b/firebird/stored-procedures/stored-procedures.md
new file mode 100644
index 0000000..94f594b
--- /dev/null
+++ b/firebird/stored-procedures/stored-procedures.md
@@ -0,0 +1,38 @@
+
+Stored Procedure is a sub-program/function which consists of a set of statements stored in the database server. Stored procedures usually consists of a generic code which can reused at multiple places. We can also pass parameters while calling the stored procedures.
+
+#### Syntax:
+```sql
+CREATE PROCEDURE sp_name(p1 datatype)
+BEGIN
+/*Stored procedure code*/
+END;
+```
+## How to call Stored procedure
+```sql
+CALL sp_name;
+```
+
+## How to delete stored procedure
+```sql
+DROP PROCEDURE sp_name;
+```
+
+#### Example:
+Consider you are writing a procedure to retrieve all the order details of a particular Customer.
+```sql
+DELIMITER $$
+CREATE PROCEDURE ORDER_SUMMARY(IN customerid Varchar(30) )
+BEGIN
+SELECT * from ORDERS where CUSTOMERID=customerid;
+END$$
+DELIMITER ;
+```
+## How to call Stored Procedure
+```sql
+CALL ORDER_SUMMARY('C10');
+```
+#### Result:
+|ORDERID|CUSTOMERID|ITEM|BILLAMOUNT|
+|---|---|---|---|
+|123|C10|Mango|5.00|
diff --git a/firebird/triggers/triggers.md b/firebird/triggers/triggers.md
new file mode 100644
index 0000000..47e9c56
--- /dev/null
+++ b/firebird/triggers/triggers.md
@@ -0,0 +1,44 @@
+
+Trigger is a set of actions which gets executed automatically when a specific event occurs like INSERT, UPDATE or DELETE.
+
+## HOW to CREATE a TRIGGER?
+
+#### Syntax:
+```sql
+CREATE TRIGGER trigger_name trigger_time trigger_event
+    ON tbl_name FOR EACH ROW [trigger_order] trigger_body
+/* where
+trigger_time: { BEFORE | AFTER }
+trigger_event: { INSERT | UPDATE | DELETE }
+trigger_order: { FOLLOWS | PRECEDES } */
+```
+#### Example:
+In the below example, we will see ORDER_HISTORY table automaticlaly gets updated after a new record is inserted into ORDERS table. This is an example of AFTER INSERT, similarly we can use triggers for INSERT/UPDATE/DELETE events and based on the requirement choose whether the update on the second table should happen BEFORE or AFTER.
+
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+--creating trigger
+CREATE TRIGGER ORD_HIS AFTER INSERT ON ORDERS  FOR EACH ROW INSERT INTO ORDER_HISTORY values( new.ORDERID,new.ITEM,new.BILL_AMOUNT,NOW());
+
+--Now insert values into ORDERS table:
+INSERT INTO ORDERS VALUES(123,'C10','MANGO',5);
+```
+Check the ORDER_HISTORY table which will yield below results:
+
+|ORDERID|ITEM|WALLETAMOUNT|DATE_OF_ORDER|
+|--- |---|---|---|
+|123|Mango|5.00|2019-10-01|
+
+## HOW to DELETE a TRIGGER?
+
+#### Syntax:
+```sql
+ DROP TRIGGER trigger_name;
+```
+
+In the above example, if we want to delete the trigger we created,
+```sql
+DROP TRIGGER  ORD_HIS;
+```
diff --git a/firebird/views/views.md b/firebird/views/views.md
new file mode 100644
index 0000000..6e45bc2
--- /dev/null
+++ b/firebird/views/views.md
@@ -0,0 +1,36 @@
+
+View is a query which is stored in the database to reuse later. Views are nothing but a pseudo table. Views can contain a complex query which retrieves data from multiple tables. So, instead of writing a complex query everytime, you can simply create a view and call it when ever required.
+
+#### Syntax:
+```sql
+Creating a View:
+CREATE VIEW View_name AS 
+Query;
+```
+
+#### How to call view
+```sql
+SELECT * FROM View_name;
+```
+#### Altering a View
+```sql
+ALTER View View_name AS 
+Query;
+```
+#### Deleting a View
+```sql
+DROP VIEW View_name;
+```
+#### Example:
+Let us consider the below two tables:
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+-- creating view
+CREATE VIEW ORD_VIEW AS SELECT O.ORDERID,O.CUSTOMERID,H.ITEM,H.DATE_OF_ORDER from ORDERS O INNER JOIN ORDER_HISTORY H; 
+
+-- to display the view
+select * from ORD_VIEW;
+```
+In this way, we can also restrict the visibility of columns of the table to the users.
\ No newline at end of file
diff --git a/ibm-db2/commands/commands.md b/ibm-db2/commands/commands.md
new file mode 100644
index 0000000..6c44c57
--- /dev/null
+++ b/ibm-db2/commands/commands.md
@@ -0,0 +1,25 @@
+This section helps you  how to write commands in DB2. Let us categorize these commands into 4 categories.
+## 1. Data Definition Language(DDL) commands
+These commands are used to define the database structures. 
+* CREATE
+* ALTER
+* TRUNCATE
+* DROP
+* RENAME
+* COMMENT
+
+## 2. Data Manipulation Language(DML) commands 
+These commands are used to manipulate the data present in the database tables.
+* INSERT
+* SELECT
+* UPDATE
+* DELETE
+## 3. Data control language(DCL) commands 
+These commands are used to define the authorizations of the database like giving right permissions to the  users.
+* GRANT
+* REVOKE
+## 4. Transaction control Language(TCL) commands
+ These commands are used to manage the changes made to the data by DML commands. 
+ * COMMIT
+ * ROLLBACK
+ * SAVEPOINT
diff --git a/ibm-db2/commands/dcl-commands.md b/ibm-db2/commands/dcl-commands.md
new file mode 100644
index 0000000..5f542d4
--- /dev/null
+++ b/ibm-db2/commands/dcl-commands.md
@@ -0,0 +1,123 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+# 1. GRANT
+
+GRANT statement is used to provide access privileges to users to access the database.
+
+## GRANT PRIVILEGES ON DATABASE
+
+### Syntax:
+
+```sql
+GRANT privileges ON DATABASE Database_name TO user/role rolename/PUBLIC [WITH GRANT OPTION];
+```
+
+Let us understand the privileges briefly
+
+|Privileges|Description|
+|--------|---------|
+|DBADM| Grants database administrator authority|
+|DBCTRL| Grants database control authority|
+|DBMAINT| Grants database maintenance authority|
+|CREATETAB| Grants the privilege to create new tables|
+|CREATETS| Grants the privilege to create new table spaces|
+|DISPLAYDB| Grants the privilege to issue the DISPLAY DATABASE command|
+|DROP|Grants the privilege to issue the DROP or ALTER DATABASE statements|
+|IMAGCOPY|Grants the privilege to run the COPY, MERGECOPY, and QUIESCE utilities and also to run MODIFY RECOVERY utility|
+|LOAD|Grants the privilege to use the LOAD utility to load tables|
+|RECOVERDB|Grants the privilege to use the RECOVER and REPORT utilities to recover table spaces and indexes|
+|REORG|Grants the privilege to use the REORG utility to reorganize table spaces and indexes|
+|REPAIR|Grants the privilege to use the REPAIR and DIAGNOSE utilities|
+|STARTDB|Grants the privilege to issue the START DATABASE command|
+|STATS|Grants the privilege to use the RUNSTATS utility to update statistics|
+|STOPDB| Grants the privilege to issue the STOP DATABASE command|
+
+### Example
+
+With this command, all the local users can execute the DISPLAY database command
+
+```sql
+   GRANT DISPLAYDB ON DATABASE DSN8D143A  TO PUBLIC;
+```
+## GRANT PRIVILEGES ON Tables and Views
+
+### Syntax
+
+```sql
+GRANT privileges ON TABLE Table_name TO user/role rolename/PUBLIC [WITH GRANT OPTION];
+```
+Let us understand the privileges briefly
+
+|Privileges|Description|
+|--------|---------|
+|ALL or ALL PRIVILEGES|Grants all table or view privileges|
+|ALTER|Grants the privilege to alter the specified table|
+|DELETE|Grants the privilege to delete rows in the specified table or view|
+|INDEX|Grants the privilege to create an index on the specified table|
+|INSERT|Grants the privilege to insert rows into the specified table or view|
+|REFERENCES (Column_name)|Grants the privilege to add or drop a referential constraint in which the specified table is a parent using only those columns that are specified in the column list as a parent key|
+|SELECT|Grants the privilege to create a view or read data from the specified table or view|
+|TRIGGER|Grants the privilege to create a trigger on the specified table
+|UPDATE|Grants the privilege to update rows in the specified table or view. |
+
+
+
+## 2. REVOKE
+
+ REVOKE statement is used to withdraw the access priviliges given to a user by GRANT statement.
+
+## REVOKE PRIVILEGES ON DATABASE
+
+### Syntax:
+
+```sql
+REVOKE privileges ON DATABASE Database_name FROM user/role rolename/PUBLIC BY user/role rolename/ALL;
+```
+
+Let us understand the privileges briefly
+
+|Privileges|Description|
+|--------|---------|
+|DBADM| Revokes the database administrator authority|
+|DBCTRL| Revokes the database control authority|
+|DBMAINT| Revokes the database maintenance authority|
+|CREATETAB| Revokes the  privilege to create new tables|
+|CREATETS| Revokes the  privilege to create new table spaces|
+|DISPLAYDB| Revokes the the privilege to issue the DISPLAY DATABASE command|
+|DROP|Revokes the the privilege to issue the DROP or ALTER DATABASE statements|
+|IMAGCOPY|Revokes the the privilege to run the COPY, MERGECOPY, and QUIESCE utilities and also to run MODIFY RECOVERY utility|
+|LOAD|Revokes the privilege to use the LOAD utility to load tables|
+|RECOVERDB|Revokes the privilege to use the RECOVER and REPORT utilities to recover table spaces and indexes|
+|REORG|Revokes the privilege to use the REORG utility to reorganize table spaces and indexes|
+|REPAIR|Revokes the privilege to use the REPAIR and DIAGNOSE utilities|
+|STARTDB|Revokes the privilege to issue the START DATABASE command|
+|STATS|Revokes the privilege to use the RUNSTATS utility to update statistics|
+|STOPDB| Revokes the privilege to issue the STOP DATABASE command|
+
+### Example
+
+With this command,  DISPLAY database command execution authorization will be revoked from all local users.
+
+```sql
+   REVOKE DISPLAYDB ON DATABASE DSN8D143A  FROM PUBLIC;
+```
+## REVOKE PRIVILEGES ON Tables and Views
+
+### Syntax
+
+```sql
+REVOKE privileges ON TABLE Table_name FROM user/role rolename/PUBLIC BY user/role rolename/ALL;
+```
+Let us understand the privileges briefly
+
+|Privileges|Description|
+|--------|---------|
+|ALL or ALL PRIVILEGES|Revokes all table or view privileges|
+|ALTER|Revokes the privilege to alter the specified table|
+|DELETE|Revokes the privilege to delete rows in the specified table or view|
+|INDEX|Revokes the privilege to create an index on the specified table|
+|INSERT|Revokes the privilege to insert rows into the specified table or view|
+|REFERENCES (Column_name)|Revokes the privilege to add or drop a referential constraint in which the specified table is a parent using only those columns that are specified in the column list as a parent key|
+|SELECT|Revokes the privilege to create a view or read data from the specified table or view|
+|TRIGGER|Revokes the privilege to create a trigger on the specified table
+|UPDATE|Revokes the privilege to update rows in the specified table or view. |
diff --git a/ibm-db2/commands/ddl-commands.md b/ibm-db2/commands/ddl-commands.md
new file mode 100644
index 0000000..a674574
--- /dev/null
+++ b/ibm-db2/commands/ddl-commands.md
@@ -0,0 +1,83 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. CREATE 
+CREATE command is used to create a table, schema or an index.
+### Syntax:
+```sql
+         CREATE TABLE table_name (
+                column1 datatype,
+                column2 datatype,
+                ....) IN DATABASE_NAME.TABLESPACE_NAME;
+``` 
+### Example:
+```sql
+        CREATE TABLE CUSTOMERS(
+            InsuranceID INT,
+            Name VARCHAR(50),
+            DOB  DATE, 
+            NIN INT, 
+            Location VARCHAR(255)
+        ) IN SAMPLEDB.SAMPLETS;
+```
+## 2. ALTER
+ ALTER command is used to add, modify or delete columns or constraints from the database table.
+        
+###    Syntax: 
+```sql 
+ALTER TABLE Table_name ADD column_name datatype;
+```
+###    Example:
+```sql
+ ALTER TABLE CUSTOMERS ADD email_id VARCHAR(50);
+```
+## 3. TRUNCATE
+ TRUNCATE command is used to delete the data present in the table but this will not delete the table.
+###    Syntax: 
+```sql
+TRUNCATE table table_name [DROP STORAGE/REUSE STORAGE] [IGNORE DELETE TRIGGERS/RESTRICT WHEN DELETE TRIGGERS] [IMMEDIATE];
+```
+### Example: 
+Empty an unused table `CUSTOMERS` regardless of any existing triggers and return its allocated space
+```sql
+TRUNCATE table CUSTOMERS DROP STORAGE IGNORE DELETE TRIGGERS;
+```
+## 4. DROP
+DROP command is used to delete the table along with its data.
+
+###    Syntax: 
+```sql 
+DROP TABLE table_name;
+```
+### Example: 
+```sql 
+DROP TABLE CUSTOMERS;
+```
+## 5. RENAME 
+RENAME command is used to rename the table name.
+
+###    Syntax:  
+```sql
+RENAME TABLE table_name to new_table_name; 
+```
+### Example: 
+```sql
+RENAME TABLE CUSTOMERS to CUSTOMERINFO;
+```
+## 6. COMMENT
+
+###  Single-Line Comments: 
+Statements starting with `--` are treated as single line comments.
+ ###   Example:
+
+ ```sql
+  --Line1;
+  ```
+
+ ###   Bracketed comments: 
+
+ Statements enclosed in `/**/` are treated as bracketed comments
+
+ ```sql
+    /* Line1,
+    Line2 */
+ ````
\ No newline at end of file
diff --git a/ibm-db2/commands/dml-commands.md b/ibm-db2/commands/dml-commands.md
new file mode 100644
index 0000000..019349f
--- /dev/null
+++ b/ibm-db2/commands/dml-commands.md
@@ -0,0 +1,52 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. INSERT  
+INSERT Statement  is used to insert new records into the database table.
+### Syntax:
+```sql
+INSERT INTO table_name (column1, column2, column3, ...) VALUES (value1, value2, value3, ...)
+```
+Note: Column names are optional.
+
+###   Example: 
+Both the below ways are correct.
+```sql
+INSERT INTO CUSTOMERS (InsuranceID, Name, DOB, NIN, Location,email_id) VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com')
+INSERT INTO CUSTOMERS VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com')
+```
+## 2. SELECT 
+Select statement is used to select data from database tables.
+
+###   Syntax:
+```sql
+SELECT column1, column2, ...
+FROM table_name
+[where condition]
+```
+### Example: 
+```sql
+SELECT * FROM CUSTOMERS 
+```    
+## 3. UPDATE 
+UPDATE statement is used to modify the existing values of records present in the database table.
+   ### Syntax:
+```sql
+ UPDATE table_name
+SET column1 = value1, column2 = value2, ...
+WHERE condition 
+```
+ ###   Example: 
+ ```sql
+ UPDATE CUSTOMERS SET email_id = 'mango.lo@xyz.com' WHERE InsuranceID='123'
+```
+## 4. DELETE 
+DELETE statement is used to delete the existing records present in the database table.
+
+###    Syntax: 
+```sql 
+DELETE FROM table_name where condition
+```
+###    Example: 
+```sql
+DELETE FROM CUSTOMERS where InsuranceID='123'
+```
\ No newline at end of file
diff --git a/ibm-db2/commands/tcl-commands.md b/ibm-db2/commands/tcl-commands.md
new file mode 100644
index 0000000..d99f866
--- /dev/null
+++ b/ibm-db2/commands/tcl-commands.md
@@ -0,0 +1,27 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+## 1. COMMIT
+COMMIT statement is used to save all the changes made to the database in the current transaction.
+### Syntax: 
+```sql
+COMMIT;
+```
+
+## 2. ROLLBACK  
+ROLLBACK statement is used to revert the changes made to a database table in the current transaction.
+### Syntax:
+```sql
+ROLLBACK;
+```
+## 3. SAVEPOINT
+SAVEPOINT statement is used to define a identifier which points to the named transaction savepoint.
+### Syntax:
+```sql
+SAVEPOINT savepoint_name;
+ROLLBACK TO SAVEPOINT savepoint_name; 
+```
+How to delete a savepoint:
+```sql
+RELEASE SAVEPOINT savepoint_name;
+```
+
diff --git a/ibm-db2/constraints/constraints.md b/ibm-db2/constraints/constraints.md
new file mode 100644
index 0000000..5934382
--- /dev/null
+++ b/ibm-db2/constraints/constraints.md
@@ -0,0 +1,71 @@
+
+Constraint is used to define rules on what values are inserted in the table. Constraint plays an important role in ensuring the accuracy and integrity of the data inserted into a table.
+
+Commonly used constraints are as follows:
+## 1. NOT NULL 
+If a column is specified as NOT NULL while defining the table, then the column must not have NULL values while inserting data into the table.
+
+### Syntax
+
+```sql
+create table table_name(column_name data_type not null,..) 
+```
+## 2. UNIQUE
+ If a column is specified as UNIQUE while defining the table, then the column must have different values for all the rows present in the table.
+
+ ### Syntax
+
+```sql
+create table table_name(column_name data_type not null unique,..) 
+```
+## 3. PRIMARY KEY 
+Primary key is a column or set of columns which is used to uniquely identify a row in the database table. In the below customer table, You can either choose InsuranceID or National Insurance Number(NIN) as primary keys, but Insurance ID is preferable as NIN can be considered as personal information.
+#### Table 1: 
+    customer(InsuranceID, Name, DOB, NIN, Location)
+#### Table 2: 
+    patient(HospitalID, Name, DOB, InsuranceID)
+
+### Syntax
+
+```sql
+create table table_name(column_name data_type not null,..,
+primary key(column_name)) 
+```
+## 4. FOREIGN KEY 
+Foreign key is a column in one table which references a primary key in other table. In the above two tables, InsuranceID of patient table can be foreign key which can be referenced to InsuranceID in customer Table.
+
+### Syntax
+```sql
+create table table_name(column_name data_type,...,constraint 
+constraint_name foreign key (column_name)  
+references ref_table_name (ref_column_name)) 
+```
+## 5. CHECK 
+checks for a specific condition while inserting data into the table. 
+
+### Syntax
+```sql
+create table table_name(column_name data_type,..,  
+primary key (),                                                       
+constraint  check (condition))
+```
+### 6. DEFAULT 
+If a value is not specified for a column whose default value is mentioned while creating the table, then the default value is inserted while inserting data.
+
+### Syntax
+
+```sql
+create table table_name(column_name data_type DEFAULT value,..) 
+```
+Note:
+1. If value is not specified then default values are as below for these data types
+    * Numeric columns : 0
+    * character or graphic fixed-length strings: blank
+    * binary fixed-length strings: a set of hexadecimal zeros
+    * datetime columns: current value of the associated special register
+2. Value can be any of the below values as the default value:
+    * a constant
+    * NULL
+    * SESSION_USER
+    * CURRENT_SQLID
+    
\ No newline at end of file
diff --git a/ibm-db2/data-types/data-types.md b/ibm-db2/data-types/data-types.md
new file mode 100644
index 0000000..df9daf3
--- /dev/null
+++ b/ibm-db2/data-types/data-types.md
@@ -0,0 +1,45 @@
+# String data types
+
+| Data Type |Description| Maximum Size Limit |
+| --------- | ----------|--------------------|
+|CHARACTER(length)|Fixed length string, length specifies the number of characters to be stored|255|
+|VARCHAR(length)|Variable length string,length specifies the number of characters to be stored|255|
+|CLOB(length)|Variable length string,length specifies the number of characters to be stored|2,147,483,647|
+|GRAPHIC(length)|Fixed-length graphic strings, length specifies the number of double-byte characters to be stored|128|
+|VARGRAPHIC(length)|Variable length string,length specifies the number of double-byte characters to be stored|16352| 
+| DBCLOB(length) |Variable length string,length specifies the number of double-byte characters to be stored | 1,073,741,824 |
+|BINARY(length)|Fixed-length or variable-length binary strings , length specifies the number of binary characters to be stored|255|
+|VARBINARY(length)|Variable binary string,length specifies the number of binary characters to be stored|32704|
+| BLOB(length) | Variable length string, length specifies the number of bytes to be stored  |2,147,483,647 |
+
+
+
+# Data and time data types
+
+
+| Data type | Format | Range |
+| ---------- | ------ | ----- |
+| DATE | 'YYYY-MM-DD' | 0001-01-01 to 9999-12-31 |
+| TIMESTAMP | 'YYYY-MM-DD-hh.mm.ss.fraction' | 0001-01-01-00.00.00.000000000 to 9999-12-31-24.00.00.000000000 | 
+| TIME | 'hh:mm:ss' |00.00.00 to 24.00.00 | 
+
+
+
+
+# Numeric data types
+
+
+| Data type | Description |
+| ---------- | ------ | 
+|SMALLINT| Small Integer with range of -32768 to +32767|
+|INT| Large Integer with range of  -2147483648 to +2147483647|
+|BIGINT| A very large Integer with range of  range is -2147483648 to +2147483647|
+|DECIMAL or NUMERIC| A packed decimal number with range of 1 - 10³¹ to 10³¹ - 1|
+|DECFLOAT|A decimal floating-point number with range is either 16 or 34 digits of precision|
+|REAL|This is a short floating-point number of 32 bits with range  approximately -7.2E+75 to 7.2E+75|
+|DOUBLE|This is a long floating-point number of 64 bits with range  approximately -7.2E+75 to 7.2E+75 |
+
+# XML Data type
+
+The XML data type is used to define columns of a table can store XML values.
+The size of an XML value has no architectural limit but serialized XML data that is stored in or retrieved from an XML column is limited to 2 GB.
\ No newline at end of file
diff --git a/ibm-db2/index.json b/ibm-db2/index.json
new file mode 100644
index 0000000..770ad76
--- /dev/null
+++ b/ibm-db2/index.json
@@ -0,0 +1,120 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title" : "About IBM DB2",
+                "path" : "about-ibmdb2"
+            },
+            {
+                "title" : "DB2 instances",
+                "path" : "db2-instance"
+
+            },
+            {
+                "title" : "Installation on Windows and Linux",
+                "path" : "installation"
+
+            }
+
+        ]
+    },
+    {
+        "title": "Data Types",
+        "path": "data-types",
+        "links": [
+            {
+                "title": "Data Types",
+                 "path": "data-types"
+            }
+        ]
+    },
+    
+    {
+        "title": "Constraints",
+        "path": "constraints",
+        "links": [
+            {
+                "title": "Constraints",
+                "path": "constraints"
+            }
+        ]
+    },
+    {
+        "title": "Commands",
+        "path": "commands",
+        "links": [
+            {
+                "title": "Commands",
+                "path": "commands"
+            },
+            {
+                "title": "DDL Commands",
+                "path": "ddl-commands"
+            },
+            {
+                "title": "DML Commands",
+                "path": "dml-commands"
+            },
+            {
+                "title": "DCL Commands",
+                "path": "dcl-commands"
+            },
+            {
+                "title": "TCL Commands",
+                "path": "tcl-commands"
+            }
+        ]
+    },
+    {
+        "title": "Joins",
+        "path": "joins",
+        "links": [
+            {
+                "title": "Joins",
+                "path": "joins"
+            }
+        ]
+    },
+    {
+        "title": "Indexes",
+        "path": "indexes",
+        "links": [
+            {
+                "title": "Indexes",
+                "path": "indexes"
+            }
+        ]
+    },
+    {
+        "title": "Triggers",
+        "path": "triggers",
+        "links": [
+            {
+                "title": "Triggers",
+                "path": "triggers"
+            }
+        ]
+    },
+    {
+        "title": "Views",
+        "path": "views",
+        "links": [
+            {
+                "title": "Views",
+                "path": "views"
+            }
+        ]
+    },
+    {
+        "title": "Stored Procedures",
+        "path": "stored-procedures",
+        "links": [
+            {
+                "title": "Stored Procedures",
+                "path": "stored-procedures"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/ibm-db2/indexes/indexes.md b/ibm-db2/indexes/indexes.md
new file mode 100644
index 0000000..78b826b
--- /dev/null
+++ b/ibm-db2/indexes/indexes.md
@@ -0,0 +1,18 @@
+In simpler terms, we can say INDEX is a pointer to some data in a table. INDEXES are very useful in attaining higher performance especially for large databases as it speeds up your search query execution.
+Unindexed tables take more time when compared to Indexed tables. For example, you can consider the synopsis of a book as an example for Indexes. You can easily search for a particular topic by referring it's page number in synopsis.  
+
+
+## 1. CREATE INDEX
+CREATE INDEX is used to create an index. INDEXes can be created on multiple columns(if required) of a table. Search query corresponding to iNDEX column will be much faster than on other columns.
+
+### Syntax:
+```sql
+  CREATE INDEX index_name on table_name(column_name);
+```
+
+## 2. DROP INDEX
+DROP INDEX is used to delete an existing index
+### Syntax
+```sql
+DROP INDEX index_name;
+```
\ No newline at end of file
diff --git a/ibm-db2/introduction/about-ibmdb2.md b/ibm-db2/introduction/about-ibmdb2.md
new file mode 100644
index 0000000..7dabd6c
--- /dev/null
+++ b/ibm-db2/introduction/about-ibmdb2.md
@@ -0,0 +1,10 @@
+IBM Db2 is a multi-workload relational  data management solution which has high performance, flexibility, and security.
+
+## Key Features
+
+* with 11.5, IBM DB2 includes AI functionality, Leverage support for data science tools
+* Uses Machine learning for  load balancing and query optimization.
+* Supports multiple languages like NoSQL, pureXML, Graph and JSON. Build in Java, .Net, Ruby, Python, Perl and more.
+* High performance with in-memory technology, storage optimization and workload management.
+* Highly secure.
+* supports Linux/Unix, Windows, MacOS and Z/OS as well.
diff --git a/ibm-db2/introduction/db2-instance.md b/ibm-db2/introduction/db2-instance.md
new file mode 100644
index 0000000..2615dfb
--- /dev/null
+++ b/ibm-db2/introduction/db2-instance.md
@@ -0,0 +1,98 @@
+DB2 instance is a logical environment for a IBM DB2 database Manager. A physical machine can contain multiple DB2 instances. A DB2 instance can contain multiple databases.
+
+## DB2 instance directory contents:
+
+* Database Manager Configuration file
+* System Database Directory
+* Node Directory
+* Node Configuration File [db2nodes.cfg] and
+* Debugging files, dump files
+
+## How to create DB2 instance 
+1. Login as root
+
+2. create a seperate operating system user for the instance you want to create.
+    ```sh
+    useradd -u <ID> -g <group name> -m -d <user location> <user name> 
+    -p <password>
+    ```
+3. Go to DB2 instance directory
+    ```sh
+    cd /opt/ibm/db2/v10.1/instance 
+    ```
+4. create DB2 instance
+    ```sh
+    ./db2icrt -s ese -u <inst id> <instance name>
+    ```
+5. Verify using list the instances present
+    ```sh
+    db2ilist 
+    ```
+
+## How to update a DB2 instance
+
+ This command is used to update the DB2 instance with same version release. 
+
+ Note: Stop the instance before updating it
+
+### Syntax
+
+```sh
+#To update the instance in normal mode
+db2iupdt <inst_name>
+
+#To update the instance in debugging mode
+db2iupdt -D <inst_name> 
+```
+
+## How to upgrade a DB2 instance
+
+This command is used to upgrade the current version of DB2 instance to a higher version.
+Navigate to DB2DIR/instance directory on UNIX/Linux.
+
+### Syntax
+
+```sh
+db2iupgrade -d -k -u <inst_username> <inst_name>
+```
+Note:
+```
+-d: debbuging mode
+-k: to keep the pre-upgrade version in the DB2copy if it's supported.
+```
+
+
+## How to delete a DB2 instance
+This command is used to delete a DB2 instance. Navigate to DB2_installation_folder/instance directory on Unix/Linux.
+
+### Syntax
+```sh
+db2idrop -u <inst_username> <inst_name> 
+```
+
+## Commands
+1. Get Instance
+ This command shows the name of the active DB2 instance.
+    ### Syntax
+    ```sh
+    db2 get instance 
+    ```
+2. Set Instance
+This command is issued to refer the instance name before start or stop of the database manager of a DB2 instance
+    ### Syntax
+    ```sh
+    set db2instance=<instance_name>
+    ```
+3. Start a DB2 instance
+This command is used to start a DB2 instance
+    ### Syntax
+    ```sh
+    db2start
+    ```
+4. Stop a DB2 instance
+This command is used to stop a DB2 instance
+    ### Syntax
+    ```sh
+    db2stop
+
+    ```
diff --git a/ibm-db2/introduction/installation.md b/ibm-db2/introduction/installation.md
new file mode 100644
index 0000000..9905fd7
--- /dev/null
+++ b/ibm-db2/introduction/installation.md
@@ -0,0 +1,22 @@
+
+## Installation on Windows
+1. Download the required version of DB2 software from [IBM](https://www.ibm.com/analytics/db2?mhsrc=ibmsearch_a&mhq=db2#1833846)
+2. Extract the product installation files and run setup.exe as an admin
+3. Setup will guide you through the installation and any settings if required.
+
+## Installation on Linux
+### Using db2setup(GUI)
+1. Download the required version of DB2 software from [IBM](https://www.ibm.com/analytics/db2?mhsrc=ibmsearch_a&mhq=db2#1833846)
+2. Untar the zip file
+```shell
+tar -xvf DB2_downloaded_filename
+```
+3. set the DISPLAY variable to run the GUI installation setup.
+```shell
+EXPORT DISPLAY=:0.0
+```
+4. Go to Server directory and run the db2setup file
+```shell
+./db2setup
+```
+5. Now the Setup will guide you through the installation and any settings if required.
diff --git a/ibm-db2/joins/joins.md b/ibm-db2/joins/joins.md
new file mode 100644
index 0000000..55dd8ec
--- /dev/null
+++ b/ibm-db2/joins/joins.md
@@ -0,0 +1,90 @@
+
+Joins are used along with SELECT statement when ever there is a need to retrieve data from multiple tables.
+
+Commonly used Joins in MariaDB are as follows:
+
+1. INNER JOIN
+2. LEFT JOIN
+3. RIGHT JOIN
+4. CROSS JOIN
+
+Let us consider the below tables for our understanding:
+
+
+TABLE1: 
+
+
+|A|Z|
+|---|---|
+|a|1|
+|b|4|
+|d|5|
+
+
+TABLE2:                    
+
+|A|X|
+|---|---|
+|a|10|
+|e|17|
+|d|30|
+
+## 1. INNER JOIN
+INNER JOIN combines the values from both the tables based on matching criteria.
+
+Below is a basic example of how inner join works:
+```sql
+SELECT * FROM TABLE1 INNER JOIN TABLE2 where TABLE1.A=TABLE2.A;
+```
+
+#### Result:
+
+|A|Z|A|X|
+|--|--|--|--|
+|a|1|a|10|
+|d|5|d|30|
+
+## 2. LEFT JOIN
+LEFT JOIN returns all the values from the Left hand table of the condition and returns only the matching values from the second table. The unmatched row values of right hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 LEFT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|b|4|null|null|
+
+## 3. RIGHT JOIN
+RIGHT JOIN returns all the values from the right hand table of the condition and returns only the matching values from the left hand table. The unmatched row values of left hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 RIGHT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|null|null|e|17|
+
+## 4. CROSS JOIN
+CROSS JOIN matches each row of the first table with every row of the second table.
+```sql
+SELECT A,Z,X from TABLE1 CROSS JOIN TABLE2;
+```
+#### Result:
+
+|A|Z|X|
+|---|---|---|
+|a|1|10|
+|a|1|17|
+|a|1|30|
+|b|4|10|
+|b|4|17|
+|b|4|30|
+|d|5|10|
+|d|5|17|
+|d|5|30|
diff --git a/ibm-db2/stored-procedures/stored-procedures.md b/ibm-db2/stored-procedures/stored-procedures.md
new file mode 100644
index 0000000..94f594b
--- /dev/null
+++ b/ibm-db2/stored-procedures/stored-procedures.md
@@ -0,0 +1,38 @@
+
+Stored Procedure is a sub-program/function which consists of a set of statements stored in the database server. Stored procedures usually consists of a generic code which can reused at multiple places. We can also pass parameters while calling the stored procedures.
+
+#### Syntax:
+```sql
+CREATE PROCEDURE sp_name(p1 datatype)
+BEGIN
+/*Stored procedure code*/
+END;
+```
+## How to call Stored procedure
+```sql
+CALL sp_name;
+```
+
+## How to delete stored procedure
+```sql
+DROP PROCEDURE sp_name;
+```
+
+#### Example:
+Consider you are writing a procedure to retrieve all the order details of a particular Customer.
+```sql
+DELIMITER $$
+CREATE PROCEDURE ORDER_SUMMARY(IN customerid Varchar(30) )
+BEGIN
+SELECT * from ORDERS where CUSTOMERID=customerid;
+END$$
+DELIMITER ;
+```
+## How to call Stored Procedure
+```sql
+CALL ORDER_SUMMARY('C10');
+```
+#### Result:
+|ORDERID|CUSTOMERID|ITEM|BILLAMOUNT|
+|---|---|---|---|
+|123|C10|Mango|5.00|
diff --git a/ibm-db2/triggers/triggers.md b/ibm-db2/triggers/triggers.md
new file mode 100644
index 0000000..3e9cc4d
--- /dev/null
+++ b/ibm-db2/triggers/triggers.md
@@ -0,0 +1,40 @@
+
+Trigger is a set of actions which gets executed automatically when a specific event occurs like INSERT, UPDATE or DELETE.
+
+## HOW to CREATE a TRIGGER?
+
+#### Syntax:
+```sql
+CREATE TRIGGER trigger_name action
+    ON tbl_name operation triggered_action
+```
+#### Example:
+In the below example, we will see ORDER_HISTORY table automaticlaly gets updated after a new record is inserted into ORDERS table. This is an example of AFTER INSERT, similarly we can use triggers for INSERT/UPDATE/DELETE events and based on the requirement choose whether the update on the second table should happen BEFORE or AFTER.
+
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+--creating trigger
+CREATE TRIGGER ORD_HIS AFTER INSERT ON ORDERS  FOR EACH ROW INSERT INTO ORDER_HISTORY values( new.ORDERID,new.ITEM,new.BILL_AMOUNT,NOW())
+
+--Now insert values into ORDERS table:
+INSERT INTO ORDERS VALUES(123,'C10','MANGO',5);
+```
+Check the ORDER_HISTORY table which will yield below results:
+
+|ORDERID|ITEM|WALLETAMOUNT|DATE_OF_ORDER|
+|--- |---|---|---|
+|123|Mango|5.00|2019-10-01|
+
+## HOW to DELETE a TRIGGER?
+
+#### Syntax:
+```sql
+DROP TRIGGER trigger_name;
+```
+
+In the above example, if we want to delete the trigger we created,
+```sql
+DROP TRIGGER  ORD_HIS;
+```
diff --git a/ibm-db2/views/views.md b/ibm-db2/views/views.md
new file mode 100644
index 0000000..3767331
--- /dev/null
+++ b/ibm-db2/views/views.md
@@ -0,0 +1,37 @@
+
+View is a query which is stored in the database to reuse later. Views are nothing but a pseudo table. Views can contain a complex query which retrieves data from multiple tables. So, instead of writing a complex query everytime, you can simply create a view and call it when ever required.
+
+
+#### Syntax:
+```sql
+--Creating a View:
+CREATE VIEW View_name AS 
+Query;
+```
+
+#### How to call view
+```sql
+SELECT * FROM View_name;
+```
+#### Altering a View
+```sql
+ALTER View View_name AS 
+Query;
+```
+#### Deleting a View
+```sql
+DROP VIEW View_name;
+```
+#### Example:
+Let us consider the below two tables:
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+-- creating view
+CREATE VIEW ORD_VIEW AS SELECT O.ORDERID,O.CUSTOMERID,H.ITEM,H.DATE_OF_ORDER from ORDERS O INNER JOIN ORDER_HISTORY H; 
+
+-- to display the view
+select * from ORD_VIEW;
+```
+In this way, we can also restrict the visibility of columns of the table to the users.
\ No newline at end of file
diff --git a/mariadb/commands/commands.md b/mariadb/commands/commands.md
new file mode 100644
index 0000000..3e59c57
--- /dev/null
+++ b/mariadb/commands/commands.md
@@ -0,0 +1,25 @@
+This section helps you  how to write commands in MariaDB. Let us categorize these commands into 4 categories.
+## 1. Data Definition Language(DDL) commands
+These commands are used to define the database structures. 
+* CREATE
+* ALTER
+* TRUNCATE
+* DROP
+* RENAME
+* COMMENT
+
+## 2. Data Manipulation Language(DML) commands 
+These commands are used to manipulate the data present in the database tables.
+* INSERT
+* SELECT
+* UPDATE
+* DELETE
+## 3. Data control language(DCL) commands 
+These commands are used to define the authorizations of the database like giving right permissions to the  users.
+* GRANT
+* REVOKE
+## 4. Transaction control Language(TCL) commands
+ These commands are used to manage the changes made to the data by DML commands. 
+ * COMMIT
+ * ROLLBACK
+ * SAVEPOINT
diff --git a/mariadb/commands/dcl-commands.md b/mariadb/commands/dcl-commands.md
new file mode 100644
index 0000000..52d48d0
--- /dev/null
+++ b/mariadb/commands/dcl-commands.md
@@ -0,0 +1,57 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+# 1. GRANT
+GRANT statement is used to provide access privileges to users to access the database.
+
+## Syntax:
+```sql
+GRANT privileges ON object TO user;
+```
+
+**Note:** Privileges can be SELECT, INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, TRIGGER, CREATE, ALL. You can also specify combination of these privileges in a statement.
+
+
+### GRANT User only to access only from localhost
+
+```sql
+GRANT USAGE ON *.* TO 'username'@localhost IDENTIFIED BY 'password';
+```
+
+### GRANT User only to access from any other computer on the network
+
+```sql
+GRANT USAGE ON *.* TO 'user'@'%' IDENTIFIED BY 'password';
+```
+
+### Grant all privileges to a user to a specific database: 
+
+```sql
+GRANT ALL privileges ON `dbname`.* TO 'username';
+```
+
+
+### Grant permission to create database:
+```sql
+ALTER USER username CREATEDB;
+```
+
+
+### Grant superuser access to a user
+```sql
+ALTER USER myuser WITH SUPERUSER;
+```
+
+
+## 2. REVOKE
+ REVOKE statement is used to withdraw the access priviliges given to a user by GRANT statement.
+
+### Syntax:
+
+```sql
+REVOKE privileges ON object FROM user;
+```
+
+### Example:
+```sql
+ REVOKE DELETE, UPDATE ON ORDERS FROM customer1;
+ ```
diff --git a/mariadb/commands/ddl-commands.md b/mariadb/commands/ddl-commands.md
new file mode 100644
index 0000000..b0f44e3
--- /dev/null
+++ b/mariadb/commands/ddl-commands.md
@@ -0,0 +1,82 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. CREATE 
+CREATE command is used to create a table, schema or an index.
+### Syntax:
+```sql
+         CREATE TABLE table_name (
+                column1 datatype,
+                column2 datatype,
+                ....);
+``` 
+### Example:
+```sql
+        CREATE TABLE CUSTOMERS(
+            InsuranceID INT,
+            Name VARCHAR(50),
+            DOB  DATE, 
+            NIN INT, 
+            Location VARCHAR(255)
+        );
+```
+## 2. ALTER
+ ALTER command is used to add, modify or delete columns or constraints from the database table.
+        
+###    Syntax: 
+```sql 
+ALTER TABLE Table_name ADD column_name datatype;
+```
+###    Example:
+```sql
+ ALTER TABLE CUSTOMERS ADD email_id VARCHAR(50);
+```
+## 3. TRUNCATE
+ TRUNCATE command is used to delete the data present in the table but this will not delete the table.
+###    Syntax: 
+```sql
+TRUNCATE table table_name;
+```
+### Example: 
+```sql
+TRUNCATE table CUSTOMERS;
+```
+## 4. DROP
+DROP command is used to delete the table along with its data.
+
+###    Syntax: 
+```sql 
+DROP TABLE table_name;
+```
+### Example: 
+```sql 
+DROP TABLE CUSTOMERS;
+```
+## 5. RENAME 
+RENAME command is used to rename the table name.
+
+###    Syntax:  
+```sql
+RENAME TABLE table_name1 to new_table_name1; 
+```
+### Example: 
+```sql
+RENAME TABLE CUSTOMERS to CUSTOMERINFO;
+```
+## 6. COMMENT
+
+###  Single-Line Comments: 
+Statements starting with `--` are treated as single line comments.
+ ###   Example:
+
+ ```sql
+  --Line1;
+  ```
+
+ ###   Multi-Line comments: 
+
+ Statements enclosed in `/**/` are treated as Multi-line comments
+
+ ```sql
+    /* Line1,
+    Line2 */
+ ````
\ No newline at end of file
diff --git a/mariadb/commands/dml-commands.md b/mariadb/commands/dml-commands.md
new file mode 100644
index 0000000..e1cd3cd
--- /dev/null
+++ b/mariadb/commands/dml-commands.md
@@ -0,0 +1,54 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. INSERT  
+INSERT Statement  is used to insert new records into the database table.
+### Syntax:
+```sql
+INSERT INTO table_name (column1, column2, column3, ...) VALUES (value1, value2, value3, ...);
+```
+
+ Note: Column names are optional.
+
+ ###   Example: 
+ Both the below ways are correct.
+```sql
+     
+INSERT INTO CUSTOMERS (InsuranceID, Name, DOB, NIN, Location,email_id) VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com');
+INSERT INTO CUSTOMERS VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com'); 
+```
+## 2. SELECT 
+Select statement is used to select data from database tables.
+
+###   Syntax:
+```sql
+SELECT column1, column2, ...
+FROM table_name
+[where condition]; 
+```
+### Example: 
+```sql
+SELECT * FROM CUSTOMERS; 
+```    
+## 3. UPDATE 
+UPDATE statement is used to modify the existing values of records present in the database table.
+   ### Syntax:
+```sql
+ UPDATE table_name
+SET column1 = value1, column2 = value2, ...
+WHERE condition; 
+```
+ ###   Example: 
+ ```sql
+ UPDATE CUSTOMERS SET email_id = 'mango.lo@xyz.com' WHERE InsuranceID='123';
+```
+## 4. DELETE 
+DELETE statement is used to delete the existing records present in the database table.
+
+###    Syntax: 
+```sql 
+DELETE FROM table_name where condition;
+```
+###    Example: 
+```sql
+DELETE FROM CUSTOMERS where InsuranceID='123';
+```
\ No newline at end of file
diff --git a/mariadb/commands/tcl-commands.md b/mariadb/commands/tcl-commands.md
new file mode 100644
index 0000000..12f166b
--- /dev/null
+++ b/mariadb/commands/tcl-commands.md
@@ -0,0 +1,27 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+## 1. COMMIT
+COMMIT statement is used to save all the changes made to the database in the current transaction.
+### Syntax: 
+```sql
+COMMIT;
+```
+
+## 2. ROLLBACK  
+ROLLBACK statement is used to revert the changes made to a database table in the current transaction.
+### Syntax:
+```sql
+ROLLBACK;
+```
+## 3. SAVEPOINT
+SAVEPOINT statement is used to define a identifier which points to the named transaction savepoint.
+### Syntax:
+```sql
+SAVEPOINT savepoint_name;
+ROLLBACK TO savepoint_name; 
+```
+How to delete a savepoint:
+```sql
+RELEASE SAVEPOINT savepoint_name;
+```
+
diff --git a/mariadb/constraints/constraints.md b/mariadb/constraints/constraints.md
new file mode 100644
index 0000000..d673708
--- /dev/null
+++ b/mariadb/constraints/constraints.md
@@ -0,0 +1,21 @@
+
+Constraint is used to define rules on what values are inserted in the table. Constraint plays an important role in ensuring the accuracy and integrity of the data inserted into a table.
+
+Commonly used constraints are as follows:
+## 1. NOT NULL 
+If a column is specified as NOT NULL while defining the table, then the column must not have NULL values while inserting data into the table.
+## 2. UNIQUE
+ If a column is specified as UNIQUE while defining the table, then the column must have different values for all the rows present in the table.
+## 3. PRIMARY KEY 
+Primary key is a column or set of columns which is used to uniquely identify a row in the database table. In the below customer table, You can either choose InsuranceID or National Insurance Number(NIN) as primary keys, but Insurance ID is preferable as NIN can be considered as personal information.
+#### Table 1: 
+    customer(InsuranceID, Name, DOB, NIN, Location)
+#### Table 2: 
+    patient(HospitalID, Name, DOB, InsuranceID)
+
+## 4. FOREIGN KEY 
+Foreign key is a column in one table which references a primary key in other table. In the above two tables, InsuranceID of patient table can be foreign key which can be referenced to InsuranceID in customer Table.
+## 5. CHECK 
+checks for a specific condition while inserting data into the table. 
+## 6. DEFAULT 
+If a value is not specified for a column whose default value is mentioned while creating the table, then the default value is inserted while inserting data.
diff --git a/mariadb/data-types/data-types.md b/mariadb/data-types/data-types.md
new file mode 100644
index 0000000..4393bdb
--- /dev/null
+++ b/mariadb/data-types/data-types.md
@@ -0,0 +1,49 @@
+# String data types
+
+| Data Type |Description| Maximum Size Limit |
+| --------- | ----------|--------------------|
+|CHAR(length)|Fixed length string, length specifies the number of characters to be stored|255|
+|VARCHAR(length)|Variable length string,length specifies the number of characters to be stored|255|
+|text(length)|Fixed length string, length specifies the number of characters to be stored|255|
+|binary(length)|Fixed length string, length specifies the number of characters to be stored|255| 
+| BLOB, TEXT |BLOB is for storing binary data and Text is used to store large string| 65,535(64KB) |
+| TINYTEXT | used to store short strings |255 |
+| MEDIUMTEXT| used to store larger text strings| 16,777,215(16MB) |
+| LONG TEXT |used to store very large data which can't be stored in MEDIUMTEXT |4,294,967,295(4GB) |
+
+
+# Data and time data types
+
+
+| Data type | Format | Range |
+| ---------- | ------ | ----- |
+| DATE | 'YYYY-MM-DD' | '1000-01-01' to '9999-12-31' |
+| DATETIME[(f)] | 'YYYY-MM-DD hh:mm:ss[.fraction]' | '1000-01-01 00:00:00.000000' to '9999-12-31 23:59:59.999999' | 
+| TIMESTAMP[(f)] | 'YYYY-MM-DD hh:mm:ss[.fraction]' | '1970-01-01 00:00:01.000000' UTC to '2038-01-19 03:14:07.999999' UTC | 
+| TIME[(f)] | 'hh:mm:ss[.fraction]' | '-838:59:59.000000' to '838:59:59.000000' | 
+| YEAR[(2/4)] | 'YY'/'YYYY'(default) | Year represented in 2 or 4 digits|
+
+
+Note: f refers to fractional seconds precision
+
+
+
+# Numeric data types
+
+### 1. Integer Family
+
+| Data Type |	Unsigned Min Value | Unsigned Max Value | Signed 	Min Value | Signed	Max Value |	
+| --------- | -------------------- | ------------------ | ----------------- | ----------------- |
+| INT	|0	| 4294967295 |	-2147483648 |	2147483647 |
+| SMALLINT  | 0	| 65535	| -32768 |	32767|
+| MEDIUMINT	| 0	| 16777215 |	-8388608 |	8388607 |
+| BIGINT	| 0	| 18446744073709551615| 	-9223372036854775808 |	9223372036854775807 |
+| TINYINT	| 0	| 255 |	-128 |	127 |
+
+### 2. FLOAT(d)
+where d is number of decimals. If the d value is in between  0-24 then the data type becomes FLOAT() else if the d value is >24 and <53 then the  data type becomes DOUBLE()
+
+### 3. DECIMAL(l,d)
+where l is number of digits displayed before the decimal point and d is number of digits after the decimal point. Default value is DECIMAL(10,2).
+
+
diff --git a/mariadb/index.json b/mariadb/index.json
new file mode 100644
index 0000000..1da46a8
--- /dev/null
+++ b/mariadb/index.json
@@ -0,0 +1,124 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title" : "About MariaDB",
+                "path" : "about-mariadb"
+            },
+            {
+                "title" : "Installation on MacOS, Linux and Windows",
+                "path" : "installation"
+
+            }
+        ]
+    },
+    {
+        "title": "Data Types",
+        "path": "data-types",
+        "links": [
+            {
+                "title": "Data Types",
+                 "path": "data-types"
+            }
+        ]
+    },
+    
+    {
+        "title": "Keys",
+        "path": "keys",
+        "links": [
+            {
+                "title": "Keys",
+                "path": "keys"
+            }
+        ]
+    },
+    {
+        "title": "Constraints",
+        "path": "constraints",
+        "links": [
+            {
+                "title": "Constraints",
+                "path": "constraints"
+            }
+        ]
+    },
+    {
+        "title": "Commands",
+        "path": "commands",
+        "links": [
+            {
+                "title": "Commands",
+                "path": "commands"
+            },
+            {
+                "title": "DDL Commands",
+                "path": "ddl-commands"
+            },
+            {
+                "title": "DML Commands",
+                "path": "dml-commands"
+            },
+            {
+                "title": "DCL Commands",
+                "path": "dcl-commands"
+            },
+            {
+                "title": "TCL Commands",
+                "path": "tcl-commands"
+            }
+        ]
+    },
+    {
+        "title": "Joins",
+        "path": "joins",
+        "links": [
+            {
+                "title": "Joins",
+                "path": "joins"
+            }
+        ]
+    },
+    {
+        "title": "Indexes",
+        "path": "indexes",
+        "links": [
+            {
+                "title": "Indexes",
+                "path": "indexes"
+            }
+        ]
+    },
+    {
+        "title": "Triggers",
+        "path": "triggers",
+        "links": [
+            {
+                "title": "Triggers",
+                "path": "triggers"
+            }
+        ]
+    },
+    {
+        "title": "Views",
+        "path": "views",
+        "links": [
+            {
+                "title": "Views",
+                "path": "views"
+            }
+        ]
+    },
+    {
+        "title": "Stored Procedures",
+        "path": "stored-procedures",
+        "links": [
+            {
+                "title": "Stored Procedures",
+                "path": "stored-procedures"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/mariadb/indexes/indexes.md b/mariadb/indexes/indexes.md
new file mode 100644
index 0000000..0cf3d76
--- /dev/null
+++ b/mariadb/indexes/indexes.md
@@ -0,0 +1,18 @@
+In simpler terms, we can say INDEX is a pointer to some data in a table. INDEXES are very useful in attaining higher performance especially for large databases as it speeds up your search query execution.
+Unindexed tables take more time when compared to Indexed tables. For example, you can consider the synopsis of a book as an example for Indexes. You can easily search for a particular topic by referring it's page number in synopsis.  
+
+
+## 1. CREATE INDEX
+CREATE INDEX is used to create an index. INDEXes can be created on multiple columns(if required) of a table. Search query corresponding to iNDEX column will be much faster than on other columns.
+
+### Syntax:
+```sql
+  CREATE INDEX index_name on table_name(column_name);
+```
+
+## 2. DROP INDEX
+DROP INDEX is used to delete an existing index
+### Syntax
+```sql
+DROP INDEX index_name ON table_name;
+```
\ No newline at end of file
diff --git a/mariadb/introduction/about-mariadb.md b/mariadb/introduction/about-mariadb.md
new file mode 100644
index 0000000..d86bd4c
--- /dev/null
+++ b/mariadb/introduction/about-mariadb.md
@@ -0,0 +1,13 @@
+MariaDB is a community developed, open source relational database management system. 
+
+## Key Features
+
+* Development was led by few of the original developers of MySQL
+* It's an improved version of MySQL with numerous inbuilt powerful features,security, performance improvements compared to MySQL.
+* High compatibility with MySQL and thus it has exact matching with MySQL APIs and commands with new features. 
+* New storage engine, Aria
+* MariaDB has been supported in Amazon RDS service and Microsoft Azure.
+* Intended to remain free and open source software under GPL, BSD or LGPL licenses.
+* Supports different programming languages and runs on different operating systems.
+
+
diff --git a/mariadb/introduction/installation.md b/mariadb/introduction/installation.md
new file mode 100644
index 0000000..f2ede9d
--- /dev/null
+++ b/mariadb/introduction/installation.md
@@ -0,0 +1,40 @@
+# Installation on MacOS
+
+## Using Homebrew
+1. Once you install Homebrew, execute the below command to install MariaDB
+```shell
+brew install mariadb
+```
+2. Start Maridb Server
+```shell
+mysql.server start
+```
+
+# Installation on Linux
+
+* Download the required version from [MariaDB Downloads](https://downloads.mariadb.org/)
+* MariaDB offers packages for RedHat/CentOS/Fedora and Debian/Ubuntu
+* openSUSE, Arch Linux, Mageia, Mint, Slackware distributions already include MariaDB package in their repositories.
+## How to install
+1. Login as `root`
+2. Navigate to MariaDB directory and import GnuPG signing key
+    ```shell
+    sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 0xcbcb082a1bb943db
+    ```
+4. Add MariaDB to sources.list file and add the below code in the file
+    ```shell
+    sudo add-apt-repository 'deb http://ftp.osuosl.org/pub/mariadb/repo/5.5/ubuntuprecise main'
+    ```
+5. Refresh the system using below command
+    ```shell
+    sudo apt-get update
+    ```
+6. Install MariaDB
+    ```shell
+    sudo apt-get install mariadb-server
+    ```
+
+# Installation on Windows
+
+* Installation on windows is much easier using MSI file. Download the required version from [MariaDB Downloads](https://downloads.mariadb.org/)
+* Just double click on the file and the setup will guide you through the installation and any settings if required.
\ No newline at end of file
diff --git a/mariadb/joins/joins.md b/mariadb/joins/joins.md
new file mode 100644
index 0000000..55dd8ec
--- /dev/null
+++ b/mariadb/joins/joins.md
@@ -0,0 +1,90 @@
+
+Joins are used along with SELECT statement when ever there is a need to retrieve data from multiple tables.
+
+Commonly used Joins in MariaDB are as follows:
+
+1. INNER JOIN
+2. LEFT JOIN
+3. RIGHT JOIN
+4. CROSS JOIN
+
+Let us consider the below tables for our understanding:
+
+
+TABLE1: 
+
+
+|A|Z|
+|---|---|
+|a|1|
+|b|4|
+|d|5|
+
+
+TABLE2:                    
+
+|A|X|
+|---|---|
+|a|10|
+|e|17|
+|d|30|
+
+## 1. INNER JOIN
+INNER JOIN combines the values from both the tables based on matching criteria.
+
+Below is a basic example of how inner join works:
+```sql
+SELECT * FROM TABLE1 INNER JOIN TABLE2 where TABLE1.A=TABLE2.A;
+```
+
+#### Result:
+
+|A|Z|A|X|
+|--|--|--|--|
+|a|1|a|10|
+|d|5|d|30|
+
+## 2. LEFT JOIN
+LEFT JOIN returns all the values from the Left hand table of the condition and returns only the matching values from the second table. The unmatched row values of right hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 LEFT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|b|4|null|null|
+
+## 3. RIGHT JOIN
+RIGHT JOIN returns all the values from the right hand table of the condition and returns only the matching values from the left hand table. The unmatched row values of left hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 RIGHT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|null|null|e|17|
+
+## 4. CROSS JOIN
+CROSS JOIN matches each row of the first table with every row of the second table.
+```sql
+SELECT A,Z,X from TABLE1 CROSS JOIN TABLE2;
+```
+#### Result:
+
+|A|Z|X|
+|---|---|---|
+|a|1|10|
+|a|1|17|
+|a|1|30|
+|b|4|10|
+|b|4|17|
+|b|4|30|
+|d|5|10|
+|d|5|17|
+|d|5|30|
diff --git a/mariadb/keys/keys.md b/mariadb/keys/keys.md
new file mode 100644
index 0000000..b22d58e
--- /dev/null
+++ b/mariadb/keys/keys.md
@@ -0,0 +1,23 @@
+let's consider the below two tables to understand the different keys present in MariaDB. 
+
+#### Table 1: 
+customer(InsuranceID, Name, DOB, NIN, Location)
+
+#### Table 2: 
+patient(HospitalID, Name, DOB, InsuranceID)
+
+## 1. Primary key 
+Primary key is a column or set of columns which is used to uniquely identify a row in the database table. In the above customer table, You can either choose InsuranceID or National Insurance Number(NIN) as primary keys, but Insurance ID is preferable as NIN can be considered as personal information.
+## 2. Super key 
+Super key is a superset of Primary key with other columns. In the above customer table, InsuranceID+Name can be a Super key. 
+## 3. Foreign key 
+Foreign key is a column in one table which references a primary key in other table. In the above two tables, InsuranceID of patient table can be considered as a foreign key which can be pointed to InsuranceID in customer Table.
+## 4. Unique key 
+Unique key is a column or set of columns which is used to uniquely identify a row in the database table. 
+
+Unique key is similar to primary key but differs in  below cases.
+* Unique key accepts only one null value where as primary key can never be null.
+* There can be multiple unique keys in a table where as primary key should be only one.
+* Primary key creates a clustered index where as unique key creates a non-clustered index. 
+## 5. Candidate key: 
+Candidate key is a column or set of columns which are eligible to become a primary key. In the customer table, InsuranceID and NIN can be candidate keys. As InsuranceID is a primary key,NIN can be a candidate key.
diff --git a/mariadb/stored-procedures/stored-procedures.md b/mariadb/stored-procedures/stored-procedures.md
new file mode 100644
index 0000000..94f594b
--- /dev/null
+++ b/mariadb/stored-procedures/stored-procedures.md
@@ -0,0 +1,38 @@
+
+Stored Procedure is a sub-program/function which consists of a set of statements stored in the database server. Stored procedures usually consists of a generic code which can reused at multiple places. We can also pass parameters while calling the stored procedures.
+
+#### Syntax:
+```sql
+CREATE PROCEDURE sp_name(p1 datatype)
+BEGIN
+/*Stored procedure code*/
+END;
+```
+## How to call Stored procedure
+```sql
+CALL sp_name;
+```
+
+## How to delete stored procedure
+```sql
+DROP PROCEDURE sp_name;
+```
+
+#### Example:
+Consider you are writing a procedure to retrieve all the order details of a particular Customer.
+```sql
+DELIMITER $$
+CREATE PROCEDURE ORDER_SUMMARY(IN customerid Varchar(30) )
+BEGIN
+SELECT * from ORDERS where CUSTOMERID=customerid;
+END$$
+DELIMITER ;
+```
+## How to call Stored Procedure
+```sql
+CALL ORDER_SUMMARY('C10');
+```
+#### Result:
+|ORDERID|CUSTOMERID|ITEM|BILLAMOUNT|
+|---|---|---|---|
+|123|C10|Mango|5.00|
diff --git a/mariadb/triggers/triggers.md b/mariadb/triggers/triggers.md
new file mode 100644
index 0000000..47e9c56
--- /dev/null
+++ b/mariadb/triggers/triggers.md
@@ -0,0 +1,44 @@
+
+Trigger is a set of actions which gets executed automatically when a specific event occurs like INSERT, UPDATE or DELETE.
+
+## HOW to CREATE a TRIGGER?
+
+#### Syntax:
+```sql
+CREATE TRIGGER trigger_name trigger_time trigger_event
+    ON tbl_name FOR EACH ROW [trigger_order] trigger_body
+/* where
+trigger_time: { BEFORE | AFTER }
+trigger_event: { INSERT | UPDATE | DELETE }
+trigger_order: { FOLLOWS | PRECEDES } */
+```
+#### Example:
+In the below example, we will see ORDER_HISTORY table automaticlaly gets updated after a new record is inserted into ORDERS table. This is an example of AFTER INSERT, similarly we can use triggers for INSERT/UPDATE/DELETE events and based on the requirement choose whether the update on the second table should happen BEFORE or AFTER.
+
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+--creating trigger
+CREATE TRIGGER ORD_HIS AFTER INSERT ON ORDERS  FOR EACH ROW INSERT INTO ORDER_HISTORY values( new.ORDERID,new.ITEM,new.BILL_AMOUNT,NOW());
+
+--Now insert values into ORDERS table:
+INSERT INTO ORDERS VALUES(123,'C10','MANGO',5);
+```
+Check the ORDER_HISTORY table which will yield below results:
+
+|ORDERID|ITEM|WALLETAMOUNT|DATE_OF_ORDER|
+|--- |---|---|---|
+|123|Mango|5.00|2019-10-01|
+
+## HOW to DELETE a TRIGGER?
+
+#### Syntax:
+```sql
+ DROP TRIGGER trigger_name;
+```
+
+In the above example, if we want to delete the trigger we created,
+```sql
+DROP TRIGGER  ORD_HIS;
+```
diff --git a/mariadb/views/views.md b/mariadb/views/views.md
new file mode 100644
index 0000000..6e45bc2
--- /dev/null
+++ b/mariadb/views/views.md
@@ -0,0 +1,36 @@
+
+View is a query which is stored in the database to reuse later. Views are nothing but a pseudo table. Views can contain a complex query which retrieves data from multiple tables. So, instead of writing a complex query everytime, you can simply create a view and call it when ever required.
+
+#### Syntax:
+```sql
+Creating a View:
+CREATE VIEW View_name AS 
+Query;
+```
+
+#### How to call view
+```sql
+SELECT * FROM View_name;
+```
+#### Altering a View
+```sql
+ALTER View View_name AS 
+Query;
+```
+#### Deleting a View
+```sql
+DROP VIEW View_name;
+```
+#### Example:
+Let us consider the below two tables:
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+-- creating view
+CREATE VIEW ORD_VIEW AS SELECT O.ORDERID,O.CUSTOMERID,H.ITEM,H.DATE_OF_ORDER from ORDERS O INNER JOIN ORDER_HISTORY H; 
+
+-- to display the view
+select * from ORD_VIEW;
+```
+In this way, we can also restrict the visibility of columns of the table to the users.
\ No newline at end of file
diff --git a/microsoft-sql-server/commands/commands.md b/microsoft-sql-server/commands/commands.md
new file mode 100644
index 0000000..2b988fa
--- /dev/null
+++ b/microsoft-sql-server/commands/commands.md
@@ -0,0 +1,25 @@
+This section helps you  how to write commands in MS SQL Server. Let us categorize these commands into 4 categories.
+## 1. Data Definition Language(DDL) commands
+These commands are used to define the database structures. 
+* CREATE
+* ALTER
+* TRUNCATE
+* DROP
+* RENAME
+* COMMENT
+
+## 2. Data Manipulation Language(DML) commands 
+These commands are used to manipulate the data present in the database tables.
+* INSERT
+* SELECT
+* UPDATE
+* DELETE
+## 3. Data control language(DCL) commands 
+These commands are used to define the authorizations of the database like giving right permissions to the  users.
+* GRANT
+* REVOKE
+## 4. Transaction control Language(TCL) commands
+ These commands are used to manage the changes made to the data by DML commands. 
+ * COMMIT
+ * ROLLBACK
+ * SAVEPOINT
diff --git a/microsoft-sql-server/commands/dcl-commands.md b/microsoft-sql-server/commands/dcl-commands.md
new file mode 100644
index 0000000..e191fdd
--- /dev/null
+++ b/microsoft-sql-server/commands/dcl-commands.md
@@ -0,0 +1,62 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+# 1. GRANT
+GRANT statement is used to provide access privileges to users to access the database.
+
+## Syntax:
+```sql
+GRANT privileges ON object TO user;
+```
+
+**Note:** Privileges can be SELECT, INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, TRIGGER, CREATE, ALL. You can also specify combination of these privileges in a statement.
+
+
+### GRANT Connect to Database
+
+```sql
+GRANT CONNECT ON DATABASE database_name TO username;
+```
+
+### GRANT Usage on Schema
+
+```sql
+GRANT USAGE ON SCHEMA database_name TO username;
+```
+
+### Grant access to all tables in the database: 
+
+```sql
+GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA schema_name TO username;
+GRANT ALL PRIVILEGES ON ALL SEQUENCES IN SCHEMA schema_name TO username;
+GRANT ALL PRIVILEGES ON DATABASE database_name TO username;
+```
+
+
+
+
+
+### Grant permission to create database:
+```sql
+ALTER USER username CREATEDB;
+```
+
+
+### Grant superuser access to a user
+```sql
+ALTER USER myuser WITH SUPERUSER;
+```
+
+
+## 2. REVOKE
+ REVOKE statement is used to withdraw the access priviliges given to a user by GRANT statement.
+
+### Syntax:
+
+```sql
+REVOKE privileges ON object FROM user;
+```
+
+### Example:
+```sql
+ REVOKE DELETE, UPDATE ON ORDERS FROM customer1;
+ ```
diff --git a/microsoft-sql-server/commands/ddl-commands.md b/microsoft-sql-server/commands/ddl-commands.md
new file mode 100644
index 0000000..7d8436e
--- /dev/null
+++ b/microsoft-sql-server/commands/ddl-commands.md
@@ -0,0 +1,89 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. CREATE 
+CREATE command is used to create a table, schema or an index.
+### Syntax:
+```sql
+         CREATE TABLE {database_name.schema_name.table_name. | schema_name.table_name | table_name } (
+                column1 datatype,
+                column2 datatype,
+                ....);
+``` 
+### Example:
+```sql
+        CREATE TABLE CUSTOMERS(
+            InsuranceID INT,
+            Name VARCHAR(50),
+            DOB  DATE, 
+            NIN INT, 
+            Location VARCHAR(255)
+        );
+```
+## 2. ALTER
+ ALTER command is used to add, modify or delete columns or constraints from the database table.
+        
+###    Syntax: 
+```sql 
+ALTER TABLE Table_name ADD column_name datatype;
+```
+###    Example:
+```sql
+ ALTER TABLE CUSTOMERS ADD email_id VARCHAR(50);
+```
+## 3. TRUNCATE
+ TRUNCATE command is used to delete the data present in the table but this will not delete the table.
+###    Syntax: 
+```sql
+TRUNCATE table table_name;
+```
+### Example: 
+```sql
+TRUNCATE table CUSTOMERS;
+```
+## 4. DROP
+DROP command is used to delete the table along with its data.
+
+###    Syntax: 
+```sql 
+DROP TABLE table_name;
+```
+### Example: 
+```sql 
+DROP TABLE CUSTOMERS;
+```
+## 5. RENAME 
+RENAME command is used to rename the table name.
+
+###    Syntax for renaming a table:  
+```sql
+USE dbname;
+GO
+EXEC sp_rename [ @objname = ] 'object_name' , [ @newname = ] 'new_name'   
+    [ , [ @objtype = ] 'object_type' ]
+GO
+```
+### Example: 
+```sql
+USE dbname;
+GO
+EXEC sp_rename 'CUSTOMERS' to 'CUSTOMERINFO';
+GO
+```
+## 6. COMMENT
+
+###  Single-Line Comments: 
+Statements starting with `--` are treated as single line comments.
+ ###   Example:
+
+ ```sql
+  --Line1;
+  ```
+
+ ###   Block comments: 
+
+ Statements enclosed in `/**/` are treated as Block comments
+
+ ```sql
+    /* Line1,
+    Line2 */
+ ````
\ No newline at end of file
diff --git a/microsoft-sql-server/commands/dml-commands.md b/microsoft-sql-server/commands/dml-commands.md
new file mode 100644
index 0000000..e1cd3cd
--- /dev/null
+++ b/microsoft-sql-server/commands/dml-commands.md
@@ -0,0 +1,54 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. INSERT  
+INSERT Statement  is used to insert new records into the database table.
+### Syntax:
+```sql
+INSERT INTO table_name (column1, column2, column3, ...) VALUES (value1, value2, value3, ...);
+```
+
+ Note: Column names are optional.
+
+ ###   Example: 
+ Both the below ways are correct.
+```sql
+     
+INSERT INTO CUSTOMERS (InsuranceID, Name, DOB, NIN, Location,email_id) VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com');
+INSERT INTO CUSTOMERS VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com'); 
+```
+## 2. SELECT 
+Select statement is used to select data from database tables.
+
+###   Syntax:
+```sql
+SELECT column1, column2, ...
+FROM table_name
+[where condition]; 
+```
+### Example: 
+```sql
+SELECT * FROM CUSTOMERS; 
+```    
+## 3. UPDATE 
+UPDATE statement is used to modify the existing values of records present in the database table.
+   ### Syntax:
+```sql
+ UPDATE table_name
+SET column1 = value1, column2 = value2, ...
+WHERE condition; 
+```
+ ###   Example: 
+ ```sql
+ UPDATE CUSTOMERS SET email_id = 'mango.lo@xyz.com' WHERE InsuranceID='123';
+```
+## 4. DELETE 
+DELETE statement is used to delete the existing records present in the database table.
+
+###    Syntax: 
+```sql 
+DELETE FROM table_name where condition;
+```
+###    Example: 
+```sql
+DELETE FROM CUSTOMERS where InsuranceID='123';
+```
\ No newline at end of file
diff --git a/microsoft-sql-server/commands/tcl-commands.md b/microsoft-sql-server/commands/tcl-commands.md
new file mode 100644
index 0000000..2a7440e
--- /dev/null
+++ b/microsoft-sql-server/commands/tcl-commands.md
@@ -0,0 +1,22 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+## 1. COMMIT
+COMMIT statement is used to save all the changes made to the database in the current transaction.
+### Syntax: 
+```sql
+COMMIT;
+```
+
+## 2. ROLLBACK  
+ROLLBACK statement is used to revert the changes made to a database table in the current transaction.
+### Syntax:
+```sql
+ROLLBACK;
+```
+## 3. SAVE
+SAVE statement is used to define a identifier which points to the named transaction savepoint.
+### Syntax:
+```sql
+SAVE [TRANSACTION] savepoint_name;
+```
+
diff --git a/microsoft-sql-server/constraints/constraints.md b/microsoft-sql-server/constraints/constraints.md
new file mode 100644
index 0000000..9994d5f
--- /dev/null
+++ b/microsoft-sql-server/constraints/constraints.md
@@ -0,0 +1,135 @@
+
+Constraint is used to define rules on what values are inserted in the table. Constraint plays an important role in ensuring the accuracy and integrity of the data inserted into a table.
+
+Commonly used constraints are as follows:
+## 1. NOT NULL 
+If a column is specified as NOT NULL while defining the table, then the column must not have NULL values while inserting data into the table.
+
+### Syntax
+```sql
+CREATE TABLE table_name
+( 
+  column_name1 datatype [ NULL | NOT NULL ] [ PRIMARY KEY ],
+  column_name2 datatype [ NULL | NOT NULL ],
+  ...
+);
+```
+
+### Example
+```sql
+CREATE TABLE customer
+( InsuranceID INT PRIMARY KEY,
+  Name VARCHAR(50) NOT NULL,
+  DOB DATE NOT NULL,
+  NIN VARCHAR(50),
+  Location VARCHAR(50)
+);
+```
+
+
+## 2. UNIQUE
+ If a column is specified as UNIQUE while defining the table, then the column must have different values for all the rows present in the table.
+
+### Syntax
+```sql
+CREATE TABLE table_name
+( 
+  column_name1 datatype [ NULL | NOT NULL ] [ PRIMARY KEY ],
+  column_name2 datatype [ NULL | NOT NULL ],
+  ...
+  CONSTRAINT constraint_name UNIQUE (column_name1,column_name2..)
+);
+```
+
+### Example
+```sql
+CREATE TABLE customer
+( InsuranceID INT PRIMARY KEY,
+  Name VARCHAR(50) NOT NULL,
+  DOB DATE NOT NULL,
+  NIN VARCHAR(50),
+  Location VARCHAR(50),
+    CONSTRAINT customer_unique UNIQUE (InsuranceID)
+);
+```
+
+## 3. PRIMARY KEY 
+Primary key is a column or set of columns which is used to uniquely identify a row in the database table. In the below customer table, You can either choose InsuranceID or National Insurance Number(NIN) as primary keys, but Insurance ID is preferable as NIN can be considered as personal information.
+
+### Syntax
+```sql
+CREATE TABLE table_name
+( 
+  column_name1 datatype [ NULL | NOT NULL ] [ PRIMARY KEY ],
+  column_name2 datatype [ NULL | NOT NULL ],
+  ...
+);
+```
+### Example
+```sql
+CREATE TABLE customer
+( InsuranceID INT PRIMARY KEY,
+  Name VARCHAR(50) NOT NULL,
+  DOB DATE NOT NULL,
+  NIN VARCHAR(50),
+  Location VARCHAR(50)
+);
+```
+
+## 4. FOREIGN KEY 
+Foreign key is a column in one table which references a primary key in other table. In the above two tables, InsuranceID of patient table can be foreign key which can be referenced to InsuranceID in customer Table.
+
+### Syntax
+```sql
+CREATE TABLE table_name
+(
+  column_name1 datatype ,
+  column_name2 datatype ,
+  ...
+
+  CONSTRAINT foreign_key_name
+    FOREIGN KEY (column_name1,column_name2,..)
+    REFERENCES parent_table (column_name1,column_name2,..)
+    [ ON DELETE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ]
+    [ ON UPDATE { NO ACTION | CASCADE | SET NULL | SET DEFAULT } ] 
+);
+```
+
+### Example
+```sql
+CREATE TABLE customer
+( 
+  InsuranceID INT PRIMARY KEY,  Name VARCHAR(50) NOT NULL, DOB DATE NOT NULL, NIN VARCHAR(50),
+  Location VARCHAR(50)
+);
+
+CREATE TABLE patient
+( HospitalID INT PRIMARY KEY, Name VARCHAR(50) NOT NULL, DOB DATE NOT NULL, InsuranceID  VARCHAR(50), CONSTRAINT fk_insurance FOREIGN KEY (InsuranceID) REFERENCES customer (InsuranceID)
+);
+```
+
+## 5. CHECK 
+checks for a specific condition while inserting data into the table.
+
+### Syntax
+```sql
+CREATE TABLE table_name
+( 
+  column_name1 datatype [ NULL | NOT NULL ] [ PRIMARY KEY ],
+  column_name2 datatype [ NULL | NOT NULL ],
+  ...
+  CONSTRAINT constraint_name
+  CHECK (column_name condition)
+);
+```
+### Example
+```sql
+CREATE TABLE customer
+( InsuranceID INT PRIMARY KEY,
+  Name VARCHAR(50) NOT NULL,
+  DOB DATE NOT NULL,
+  NIN INT,
+  Location VARCHAR(50)
+  CONSTRAINT customer_chk CHECK (NIN > 0)
+);
+```
diff --git a/microsoft-sql-server/data-types/date-and-time.md b/microsoft-sql-server/data-types/date-and-time.md
new file mode 100644
index 0000000..a716928
--- /dev/null
+++ b/microsoft-sql-server/data-types/date-and-time.md
@@ -0,0 +1,10 @@
+
+| Data type | Format | Range |
+| ---------- | ------ | ----- |
+| DATE | 'YYYY-MM-DD' | '0001-01-01' to '9999-12-31' |
+| DATETIME[(f)] | 'YYYY-MM-DD hh:mm:ss[.fraction]' | '1753-01-01 00:00:00.000000' to '9999-12-31 23:59:59.999999' | 
+| TIME[(f)] | 'hh:mm:ss[.fraction]' | '00:00:00.0000000' to '23:59:59.000000' | 
+| YEAR[(2/4)] | 'YY'/'YYYY'(default) | Year represented in 2 or 4 digits|
+
+
+Note: f refers to fractional seconds precision
diff --git a/microsoft-sql-server/data-types/numeric-data-types.md b/microsoft-sql-server/data-types/numeric-data-types.md
new file mode 100644
index 0000000..70b44d5
--- /dev/null
+++ b/microsoft-sql-server/data-types/numeric-data-types.md
@@ -0,0 +1,16 @@
+## Numeric Data Types
+
+| Data Type |Min Value | Max Value | Memory |	
+| --------- | ------- | -------- | -------- | 
+| INT	| −2^31 | 2^31−1 |	4 bytes |
+| SMALLINT  | −2^15	| 2^15	| 2 bytes |
+| BIGINT	| −2^63	| 2^63−1| 8 bytes |
+| TINYINT	| 0	| 255 |	1 byte |
+| DECIMAL |−10^38+1|10^381−1|5 to 17 bytes|
+| MONEY | −922,337, 203, 685,477.5808|+922,337, 203, 685,477.5807| 8 bytes|
+| FLOAT | −1.79E+308| 1.79E+308 | depends on float precision |
+| REAL | −3.40E+38 | 3.40E+38 | 4 bytes |
+
+
+
+
diff --git a/microsoft-sql-server/data-types/string-data-types.md b/microsoft-sql-server/data-types/string-data-types.md
new file mode 100644
index 0000000..d0efc0a
--- /dev/null
+++ b/microsoft-sql-server/data-types/string-data-types.md
@@ -0,0 +1,13 @@
+
+## 1. CHAR(l)
+CHAR is a fixed length string data type. Range can be from 0 to 8000 chars. Character values are stored with the specified length(l) and they are right padded with spaces.
+
+## 2. VARCHAR(l)
+VARCHAR is a variable length string data type. Range can be from 0 to 8000 chars
+
+## 3. BINARY and VARBINARY 
+BINARY and VARBINARY are very similar to CHAR and VARCHAR except that the data is stored as bytes where as the data is stored as characters for CHAR and VARCHAR. BINARY values are right padded with spaces. 
+
+## 4. TEXT 
+TEXT is variable length character string. Range can be from 0 to 2,147,483,647 chars.
+
diff --git a/microsoft-sql-server/index.json b/microsoft-sql-server/index.json
new file mode 100644
index 0000000..1db716e
--- /dev/null
+++ b/microsoft-sql-server/index.json
@@ -0,0 +1,121 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title" : "About Microsoft SQL Server",
+                "path" : "about-mssqlserver"
+            },
+            {
+                "title" : "Installation on Linux and Windows",
+                "path" : "installation"
+
+            }
+        ]
+    },
+    {
+        "title": "Data Types",
+        "path": "data-types",
+        "links": [
+            {
+                "title": "Numeric Data Types",
+                "path": "numeric-data-types"
+            },
+            {
+                "title": "Date and Time",
+                "path": "date-and-time"
+            },
+            {
+                "title": "String Data Types",
+                "path": "string-data-types"
+            }
+        ]
+    },
+    {
+        "title": "Constraints",
+        "path": "constraints",
+        "links": [
+            {
+                "title": "Constraints",
+                "path": "constraints"
+            }
+        ]
+    },
+    {
+        "title": "Microsoft SQL Server Commands",
+        "path": "commands",
+        "links": [
+            {
+                "title": "Commands",
+                "path": "commands"
+            },
+            {
+                "title": "DDL Commands",
+                "path": "ddl-commands"
+            },
+            {
+                "title": "DML Commands",
+                "path": "dml-commands"
+            },
+            {
+                "title": "DCL Commands",
+                "path": "dcl-commands"
+            },
+            {
+                "title": "TCL Commands",
+                "path": "tcl-commands"
+            }
+        ]
+    },
+    {
+        "title": "Joins",
+        "path": "joins",
+        "links": [
+            {
+                "title": "Joins",
+                "path": "joins"
+            }
+        ]
+    },
+    {
+        "title": "Indexes",
+        "path": "indexes",
+        "links": [
+            {
+                "title": "Indexes",
+                "path": "indexes"
+            }
+        ]
+    },
+    {
+        "title": "Triggers",
+        "path": "triggers",
+        "links": [
+            {
+                "title": "Triggers",
+                "path": "triggers"
+            }
+        ]
+    },
+    {
+        "title": "Views",
+        "path": "views",
+        "links": [
+            {
+                "title": "Views",
+                "path": "views"
+            }
+        ]
+    },
+    {
+        "title": "Stored Procedures",
+        "path": "stored-procedures",
+        "links": [
+            {
+                "title": "Stored Procedures",
+                "path": "stored-procedures"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/microsoft-sql-server/indexes/indexes.md b/microsoft-sql-server/indexes/indexes.md
new file mode 100644
index 0000000..eaab94e
--- /dev/null
+++ b/microsoft-sql-server/indexes/indexes.md
@@ -0,0 +1,19 @@
+In simpler terms, we can say INDEX is a pointer to some data in a table. INDEXES are very useful in attaining higher performance especially for large databases as it speeds up your search query execution.
+
+For example, you can consider the synopsis of a book as an example for Indexes. You can easily search for a particular topic by referring it's page number in synopsis.  
+
+
+## 1. CREATE INDEX
+CREATE INDEX is used to create an index. INDEXes can be created on multiple columns(if required) of a table. Search query corresponding to INDEX column will be much faster than on other columns.
+
+### Syntax:
+```sql
+CREATE INDEX index_name on table_name(column_name);
+```
+
+## 2. DROP INDEX
+DROP INDEX is used to delete an existing index
+### Syntax
+```sql
+DROP INDEX table_name.index_name;
+```
\ No newline at end of file
diff --git a/microsoft-sql-server/introduction/about-mssqlserver.md b/microsoft-sql-server/introduction/about-mssqlserver.md
new file mode 100644
index 0000000..2d1cc0d
--- /dev/null
+++ b/microsoft-sql-server/introduction/about-mssqlserver.md
@@ -0,0 +1,22 @@
+Microsoft SQL server is a popular relational database management system which is developed, distributed and supported by Microsoft. Also known as MS SQL Server.
+
+## Key Features:
+
+* MS SQL Server supports ANSI SQL, however Microsoft came up with its own propriety SQL language called Transact SQL(T-SQL).
+* SQL Server Editions
+### 1. SQL Server Enterprise
+        * For large scale and mission Critical business. 
+        * Provides High-end security, Advanced Analytics, Machine Learning, etc.
+### 2. SQL Server Standard
+        * For Mid-Tier Application and Data marts. 
+        * Includes basic reporting and analytic features
+### 3. SQL Server WEB
+        * For a low total-cost-of-ownership option for Web hosters. 
+        * Provides scalability, affordability, and manageability capabilities for small to large scale Web properties.
+### 4. SQL Server Developer
+        * For non-production environment but similar to Enterprise Edition . 
+        * For development purpose. 
+### 5. SQL Server Express
+        * Free and for small scale applications.
+
+
diff --git a/microsoft-sql-server/introduction/installation.md b/microsoft-sql-server/introduction/installation.md
new file mode 100644
index 0000000..5f487ca
--- /dev/null
+++ b/microsoft-sql-server/introduction/installation.md
@@ -0,0 +1,33 @@
+# Installation on Linux
+
+* Download  Microsoft SQL Server 2017 Red Hat repository configuration file using the below command
+
+    ```sh
+    sudo curl -o /etc/yum.repos.d/mssql-server.repo https://packages.microsoft.com/config/rhel/7/mssql-server-2017.repo
+    ```
+* Install SQL Server
+
+    ```sh
+    sudo yum install -y mssql-server
+    ```
+* After Installation gets finshed, run mssql-conf setup and set the SA password, edition etc.
+
+    ```sh
+    sudo /opt/mssql/bin/mssql-conf setup
+    ```
+* Check whether the services are running using below command
+    ```sh
+    systemctl status mssql-server
+    ```
+* Open SQL server port for allowing remote connections(Default port:1433).
+    ```sh
+    sudo firewall-cmd --zone=public --add-port=1433/tcp --permanent
+    sudo firewall-cmd --reload
+    ```
+
+
+# Installation on Windows
+
+* Download the required version of Developer edition from [SQL Server Downloads](https://www.microsoft.com/en-us/sql-server/sql-server-downloads)
+* Run the executable file and the setup will walk you through the trivial process.
+* Installation will take few minutes to complete.
diff --git a/microsoft-sql-server/joins/joins.md b/microsoft-sql-server/joins/joins.md
new file mode 100644
index 0000000..787d3f0
--- /dev/null
+++ b/microsoft-sql-server/joins/joins.md
@@ -0,0 +1,90 @@
+
+Joins are used along with SELECT statement when ever there is a need to retrieve data from multiple tables.
+
+Commonly used Joins in Microsoft SQL Server are as follows:
+
+1. INNER JOIN
+2. LEFT JOIN
+3. RIGHT JOIN
+4. CROSS JOIN
+
+Let us consider the below tables for our understanding:
+
+
+TABLE1: 
+
+
+|A|Z|
+|---|---|
+|a|1|
+|b|4|
+|d|5|
+
+
+TABLE2:                    
+
+|A|X|
+|---|---|
+|a|10|
+|e|17|
+|d|30|
+
+## 1. INNER JOIN
+INNER JOIN combines the values from both the tables based on matching criteria.
+
+Below is a basic example of how inner join works:
+```sql
+SELECT * FROM TABLE1 INNER JOIN TABLE2 where TABLE1.A=TABLE2.A;
+```
+
+#### Result:
+
+|A|Z|A|X|
+|--|--|--|--|
+|a|1|a|10|
+|d|5|d|30|
+
+## 2. LEFT JOIN
+LEFT JOIN returns all the values from the Left hand table of the condition and returns only the matching values from the second table. The unmatched row values of right hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 LEFT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|b|4|null|null|
+
+## 3. RIGHT JOIN
+RIGHT JOIN returns all the values from the right hand table of the condition and returns only the matching values from the left hand table. The unmatched row values of left hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 RIGHT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|null|null|e|17|
+
+## 4. CROSS JOIN
+CROSS JOIN matches each row of the first table with every row of the second table.
+```sql
+SELECT A,Z,X from TABLE1 CROSS JOIN TABLE2;
+```
+#### Result:
+
+|A|Z|X|
+|---|---|---|
+|a|1|10|
+|a|1|17|
+|a|1|30|
+|b|4|10|
+|b|4|17|
+|b|4|30|
+|d|5|10|
+|d|5|17|
+|d|5|30|
diff --git a/microsoft-sql-server/stored-procedures/stored-procedures.md b/microsoft-sql-server/stored-procedures/stored-procedures.md
new file mode 100644
index 0000000..d60482d
--- /dev/null
+++ b/microsoft-sql-server/stored-procedures/stored-procedures.md
@@ -0,0 +1,40 @@
+
+Stored Procedure is a sub-program/function which consists of a set of statements stored in the database server. Stored procedures usually consists of a generic code which can be reused at multiple places. We can also pass parameters while calling the stored procedures.
+
+#### Syntax:
+```sql
+CREATE PROCEDURE sp_name
+AS
+BEGIN
+/*Stored procedure code*/
+END
+GO
+```
+## How to call Stored procedure
+```sql
+CALL sp_name;
+```
+
+## How to delete stored procedure
+```sql
+DROP PROCEDURE sp_name;
+```
+
+#### Example:
+Consider you are writing a procedure to retrieve all the order details of a particular Customer.
+```sql
+DELIMITER $$
+CREATE PROCEDURE ORDER_SUMMARY(IN customerid Varchar(30) )
+BEGIN
+SELECT * from ORDERS where CUSTOMERID=customerid;
+END$$
+DELIMITER ;
+```
+## How to call Stored Procedure
+```sql
+CALL ORDER_SUMMARY('C10');
+```
+#### Result:
+|ORDERID|CUSTOMERID|ITEM|BILLAMOUNT|
+|---|---|---|---|
+|123|C10|Mango|5.00|
diff --git a/microsoft-sql-server/triggers/triggers.md b/microsoft-sql-server/triggers/triggers.md
new file mode 100644
index 0000000..29e733a
--- /dev/null
+++ b/microsoft-sql-server/triggers/triggers.md
@@ -0,0 +1,42 @@
+
+Trigger is a set of actions which gets executed automatically when a specific event occurs like INSERT, UPDATE or DELETE.
+
+## How to create a TRIGGER?
+
+#### Syntax:
+```sql
+CREATE TRIGGER trigger_name ON tbl_name
+{FOR|AFTER|INSTEAD OF} { [ INSERT ] [ , ] [ UPDATE ] [ , ] [ DELETE ] } AS
+SQL Statements;
+```
+#### Example:
+In the below example, we will see ORDER_HISTORY table automaticlaly gets updated after a new record is inserted into ORDERS table. This is an example of AFTER INSERT, similarly we can use triggers for INSERT/UPDATE/DELETE events and based on the requirement choose whether the update on the second table should happen BEFORE or AFTER.
+
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+--creating trigger
+CREATE TRIGGER ORD_HIS ON ORDERS AFTER INSERT AS
+INSERT INTO ORDER_HISTORY values( new.ORDERID,new.ITEM,new.BILL_AMOUNT,NOW());
+
+--Now insert values into ORDERS table:
+INSERT INTO ORDERS VALUES(123,'C10','MANGO',5);
+```
+Check the ORDER_HISTORY table which will yield below results:
+
+|ORDERID|ITEM|WALLETAMOUNT|DATE_OF_ORDER|
+|--- |---|---|---|
+|123|Mango|5.00|2019-10-01|
+
+## How to delete a TRIGGER?
+
+#### Syntax:
+```sql
+ DROP TRIGGER trigger_name;
+```
+
+In the above example, if we want to delete the trigger we created,
+```sql
+DROP TRIGGER  ORD_HIS;
+```
diff --git a/microsoft-sql-server/views/views.md b/microsoft-sql-server/views/views.md
new file mode 100644
index 0000000..6e45bc2
--- /dev/null
+++ b/microsoft-sql-server/views/views.md
@@ -0,0 +1,36 @@
+
+View is a query which is stored in the database to reuse later. Views are nothing but a pseudo table. Views can contain a complex query which retrieves data from multiple tables. So, instead of writing a complex query everytime, you can simply create a view and call it when ever required.
+
+#### Syntax:
+```sql
+Creating a View:
+CREATE VIEW View_name AS 
+Query;
+```
+
+#### How to call view
+```sql
+SELECT * FROM View_name;
+```
+#### Altering a View
+```sql
+ALTER View View_name AS 
+Query;
+```
+#### Deleting a View
+```sql
+DROP VIEW View_name;
+```
+#### Example:
+Let us consider the below two tables:
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+-- creating view
+CREATE VIEW ORD_VIEW AS SELECT O.ORDERID,O.CUSTOMERID,H.ITEM,H.DATE_OF_ORDER from ORDERS O INNER JOIN ORDER_HISTORY H; 
+
+-- to display the view
+select * from ORD_VIEW;
+```
+In this way, we can also restrict the visibility of columns of the table to the users.
\ No newline at end of file
diff --git a/mongodb/advanced-topics/aggregations.md b/mongodb/advanced-topics/aggregations.md
new file mode 100644
index 0000000..05e0d11
--- /dev/null
+++ b/mongodb/advanced-topics/aggregations.md
@@ -0,0 +1,25 @@
+Aggregations in MongoDB are used to process records and return computed results. There are three ways of performing aggregation operations in MongoDB
+
+1. Aggregation Pipeline
+2. Map-Reduce
+3. Single Purpose Aggregation Operations
+
+
+## Aggregation Pipeline
+In Aggregation Pipelines data enters through multi-stage pipeline and the data gets transfomed into an aggregated result. 
+
+## Map-Reduce
+Map-Reduce operations have two phases first is a mapping stage which takes a document and transform it to another with the given inputs, then the reduce stage that combines the output of Map stage
+
+## Single Purpose Aggregation Operations
+MongoDB provides three simple to use Single Purpose Aggregation Operations
+
+1. estimatedDocumentCount
+2. count
+3. distinct
+
+Examples: 
+```javascript
+db.cars.count(); // returns the total count of documents from cars collection
+db.cars.distinct('type'); // returns all distinct values if type Ex. SUV, Sedan, Hatchback
+```
\ No newline at end of file
diff --git a/mongodb/advanced-topics/indexes.md b/mongodb/advanced-topics/indexes.md
new file mode 100644
index 0000000..761fbbc
--- /dev/null
+++ b/mongodb/advanced-topics/indexes.md
@@ -0,0 +1,18 @@
+Indexes are used to run queries more efficiently in MongoDB. 
+
+## Creating Indexes
+For example you are going to make queries based on `companyName` in your queries then you can create an index on `companyName` field. 
+
+```javascript
+db.collection.createIndex( { companyName: 1 } )
+```
+
+## Index Types
+MongoDB provides different types of indexes to support different usecases. Following are the different types of indexes MongoDB provides
+
+1. Single Field - ascending/descending indexes on a single field
+2. Compound Index - to create indexes on multiple fields
+3. Multikey Index - to create indexes on array type fields 
+4. Geospatial Index - to perform queries on geospatial data
+5. Text Indexes - For text search
+6. Hashed Indexes - For hash based sharding
diff --git a/mongodb/crud-operations/creating-data.md b/mongodb/crud-operations/creating-data.md
new file mode 100644
index 0000000..7b2cf33
--- /dev/null
+++ b/mongodb/crud-operations/creating-data.md
@@ -0,0 +1,37 @@
+### Inserting a single Document
+
+We can use `insertOne()` to insert a single Document.
+
+```javascript
+db.cars.insertOne({
+    companyName: "Volvo",
+    model: "XC-90",
+    type: "SUV",
+    year: "2019",
+    colors: ["black", "white", "silver"],
+    specifications: {
+        length: 4950,
+        width: 2140,
+        height: 1776
+    }
+})
+```
+
+### Inserting Multiple Documents
+
+`insertMany()` is used to insert multiple documents 
+
+```javascript
+db.variants.insertMany([
+   {companyName: "Volvo", model: "XC-90", variants: "T8 Inscription"},
+   {companyName: "Volvo", model: "XC-90", variants: "T8 Excellence"}
+])
+```
+
+### The `_id` field
+
+MongoDB needs a unique `_id` field for each and every document we save to a collection. If user does not provide a `_id` field then MongoDB generates an unique ObjectId for the `_id` field.
+
+
+### Deprecated `save` Method
+MongoDB also have save method to insert/ update documents. This need to avoided as its deprecated
diff --git a/mongodb/crud-operations/creating-database.md b/mongodb/crud-operations/creating-database.md
new file mode 100644
index 0000000..d51d1e0
--- /dev/null
+++ b/mongodb/crud-operations/creating-database.md
@@ -0,0 +1,5 @@
+We can create a new database by running the following command. This creates a new database if not already exists.
+
+```sh
+use <db_name>
+```
\ No newline at end of file
diff --git a/mongodb/crud-operations/deleting-data.md b/mongodb/crud-operations/deleting-data.md
new file mode 100644
index 0000000..b4d1355
--- /dev/null
+++ b/mongodb/crud-operations/deleting-data.md
@@ -0,0 +1,46 @@
+`deleteOne` and `deleteMany` are used to delete existing documents in MongoDB.
+
+## Deleting a single document
+`deleteOne` is used to delete a single document in a collection
+
+#### Syntax
+
+```javascript
+db.cars.deleteOne({
+    <query>
+});
+```
+
+#### Example
+
+```javascript
+db.cars.deleteOne({
+    {
+        companyName: "Volvo",
+        model: "XC-90"
+    }
+});
+```
+
+
+## Deleting multiple documents
+
+`deleteMany` is used to delete multiple documents in a collection
+
+#### Syntax
+
+```javascript
+db.cars.deleteMany({
+    <query>
+});
+```
+
+#### Example
+
+```javascript
+db.cars.deleteMany({
+    {
+        companyName: "Volvo"
+    }
+});
+```
\ No newline at end of file
diff --git a/mongodb/crud-operations/reading-data.md b/mongodb/crud-operations/reading-data.md
new file mode 100644
index 0000000..e12a67b
--- /dev/null
+++ b/mongodb/crud-operations/reading-data.md
@@ -0,0 +1,28 @@
+## Reading all documents from a collection
+`find` is used to read documents from a MongoDB collection. This takes a query as input. If an empty query provided then it returns all the documents from a collection. 
+
+```javascript
+db.cars.find({});
+```
+
+## Reading documents with matching condition
+If a query passed to find method, MongoDB returns documents matching that filter condition
+
+```javascript
+db.cars.find({ type: "SUV" });
+```
+
+## Reading a single document using `findOne`
+`findOne` is used to return a single document matching the filter condition
+
+```javascript
+db.cars.find({ type: "SUV" });
+```
+
+## Matching multiple values
+`in` operater is used to match multiple values
+
+```javascript
+db.cars.find({ type: {$in : ["SUV", "Sedan"]} });
+```
+
diff --git a/mongodb/crud-operations/updating-data.md b/mongodb/crud-operations/updating-data.md
new file mode 100644
index 0000000..7347e5e
--- /dev/null
+++ b/mongodb/crud-operations/updating-data.md
@@ -0,0 +1,55 @@
+`updateOne` and `updateMany` are used to update existing documents in MongoDB.
+
+## Updating a single document
+`updateOne` is used to update a single document in a collection
+
+#### Syntax
+
+```javascript
+db.cars.updateOne({
+    <query>,
+    <updated_data>
+});
+```
+
+#### Example
+
+```javascript
+db.cars.updateOne({
+    {
+        companyName: "Volvo",
+        model: "XC-90"
+    },
+    {
+        $set: {type: "SUV"}
+    }
+});
+```
+
+Note: `$set` operator updates the given fields to the new values
+
+## Updating multiple documents
+
+`updateMany` is used to update multiple documents in a collection
+
+#### Syntax
+
+```javascript
+db.cars.updateMany({
+    <query>,
+    <updated_data>
+});
+```
+
+#### Example
+
+```javascript
+db.cars.updateMany({
+    {
+        companyName: "Volvo"
+    },
+    {
+        $set: { type: "SUV" }
+    }
+});
+```
\ No newline at end of file
diff --git a/mongodb/index.json b/mongodb/index.json
new file mode 100644
index 0000000..6877919
--- /dev/null
+++ b/mongodb/index.json
@@ -0,0 +1,60 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title": "What is MongoDB",
+                "path": "what-is-mongodb"
+            },
+            {
+                "title": "Key Concepts",
+                "path": "key-concepts"
+            },
+            {
+                "title": "Installation",
+                "path": "installation"
+            }
+        ]
+    },
+    {
+        "title": "CRUD Operations",
+        "path": "crud-operations",
+        "links": [
+            {
+                "title": "Creating Database",
+                "path": "creating-database"
+            },
+            {
+                "title": "Inserting Data",
+                "path": "creating-data"
+            },
+            {
+                "title": "Reading Data",
+                "path": "reading-data"
+            },
+            {
+                "title": "Updating Data",
+                "path": "updating-data"
+            },
+            {
+                "title": "Deleting Data",
+                "path": "deleting-data"
+            }
+        ]
+    },
+    {
+        "title": "Advanced Topics",
+        "path": "advanced-topics",
+        "links": [
+            {
+                "title": "Aggregations",
+                "path": "aggregations"
+            },
+            {
+                "title": "Indexes",
+                "path": "indexes"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/mongodb/introduction/installation.md b/mongodb/introduction/installation.md
new file mode 100644
index 0000000..a4d3bf7
--- /dev/null
+++ b/mongodb/introduction/installation.md
@@ -0,0 +1,34 @@
+MongoDB is available as a cloud offering which is called MongoDB atlas, use can use it with out you maintaining it. But if you choose to Install and manage by your own you can do that. Following sections shows you how to install it in different platforms.
+
+## Installing on MacOS
+
+Easyes way to get MongoDB installed on MacOS is via Homebrew. You can run the following command to install MongoDB in MacOS
+```sh
+brew install mongodb
+```
+
+## Installing on Windows
+
+MongoDB is provided as an MSI installer package for Windows, we can get it from the following link
+https://www.mongodb.com/download-center/community
+After downloading, we need to run the installer and install it on Windows. 
+
+## Installing on Ubuntu
+We can install MongoDB on Ubuntu by running the following series of commands
+
+
+### For Ubuntu 16.04
+```sh
+wget -qO - https://www.mongodb.org/static/pgp/server-4.2.asc | sudo apt-key add -
+echo "deb [ arch=amd64 ] https://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/4.2 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-4.2.list
+sudo apt-get update
+sudo apt-get install -y mongodb-org
+```
+
+### For Ubuntu 18.04
+```sh
+wget -qO - https://www.mongodb.org/static/pgp/server-4.2.asc | sudo apt-key add -
+echo "deb [ arch=amd64 ] https://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/4.2 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-4.2.list
+sudo apt-get update
+sudo apt-get install -y mongodb-org
+```
\ No newline at end of file
diff --git a/mongodb/introduction/key-concepts.md b/mongodb/introduction/key-concepts.md
new file mode 100644
index 0000000..7061a8b
--- /dev/null
+++ b/mongodb/introduction/key-concepts.md
@@ -0,0 +1,24 @@
+## Document Oriented
+Document oriented databases are different from traditional RDBMS. 
+Document oriented databases contain documents these are like rows in RDBMS which stores the data. These documents can be complex and have multilevel data. For example, a user document can have a list of roles which is an array of Strings and also can contain address which is another object with its own fields like the dollowing example 
+
+```json
+{
+    "id": 1,
+    "name": "Tom",
+    "roles": ["admin", "manager"],
+    "address" : {
+        "street": "xxxx",
+        "zipcode": 12345,
+        "country": "US"
+    }
+}
+```
+## No Schema
+Mongo DB is a schema less database, that means user don't have to define a schema before start inserting the data. And also it is flexible i.e we can introduce new fields without defining them anywhere.
+
+## database, collection, fields
+Just like any other database users can create multiple databases in MongoDB, Database contains collections, collections contains documents. The document is made up of fields and its data.
+
+## Mongo Shell 
+Mongo shell is a utility provided by MongoDB. It is a CLI interface to interact with MongoDB.
diff --git a/mongodb/introduction/what-is-mongodb.md b/mongodb/introduction/what-is-mongodb.md
new file mode 100644
index 0000000..9fced93
--- /dev/null
+++ b/mongodb/introduction/what-is-mongodb.md
@@ -0,0 +1,2 @@
+MongoDB is a opensource document-oriented NoSQL database. MongoDB is available as free Community version, also as an Enterprise version. 
+MongoDB also provides it's own cloud offering called Mongo Atlas where users can try MongoDB which out installing or managing it on their own. 
diff --git a/mysql/commands/commands.md b/mysql/commands/commands.md
new file mode 100644
index 0000000..2605193
--- /dev/null
+++ b/mysql/commands/commands.md
@@ -0,0 +1,25 @@
+This section helps you  how to write commands in MySQL. Let us categorize these commands into 4 categories.
+## 1. Data Definition Language(DDL) commands
+These commands are used to define the database structures. 
+* CREATE
+* ALTER
+* TRUNCATE
+* DROP
+* RENAME
+* COMMENT
+
+## 2. Data Manipulation Language(DML) commands 
+These commands are used to manipulate the data present in the database tables.
+* INSERT
+* SELECT
+* UPDATE
+* DELETE
+## 3. Data control language(DCL) commands 
+These commands are used to define the authorizations of the database like giving right permissions to the  users.
+* GRANT
+* REVOKE
+## 4. Transaction control Language(TCL) commands
+ These commands are used to manage the changes made to the data by DML commands. 
+ * COMMIT
+ * ROLLBACK
+ * SAVEPOINT
diff --git a/mysql/commands/dcl-commands.md b/mysql/commands/dcl-commands.md
new file mode 100644
index 0000000..e191fdd
--- /dev/null
+++ b/mysql/commands/dcl-commands.md
@@ -0,0 +1,62 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+# 1. GRANT
+GRANT statement is used to provide access privileges to users to access the database.
+
+## Syntax:
+```sql
+GRANT privileges ON object TO user;
+```
+
+**Note:** Privileges can be SELECT, INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, TRIGGER, CREATE, ALL. You can also specify combination of these privileges in a statement.
+
+
+### GRANT Connect to Database
+
+```sql
+GRANT CONNECT ON DATABASE database_name TO username;
+```
+
+### GRANT Usage on Schema
+
+```sql
+GRANT USAGE ON SCHEMA database_name TO username;
+```
+
+### Grant access to all tables in the database: 
+
+```sql
+GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA schema_name TO username;
+GRANT ALL PRIVILEGES ON ALL SEQUENCES IN SCHEMA schema_name TO username;
+GRANT ALL PRIVILEGES ON DATABASE database_name TO username;
+```
+
+
+
+
+
+### Grant permission to create database:
+```sql
+ALTER USER username CREATEDB;
+```
+
+
+### Grant superuser access to a user
+```sql
+ALTER USER myuser WITH SUPERUSER;
+```
+
+
+## 2. REVOKE
+ REVOKE statement is used to withdraw the access priviliges given to a user by GRANT statement.
+
+### Syntax:
+
+```sql
+REVOKE privileges ON object FROM user;
+```
+
+### Example:
+```sql
+ REVOKE DELETE, UPDATE ON ORDERS FROM customer1;
+ ```
diff --git a/mysql/commands/ddl-commands.md b/mysql/commands/ddl-commands.md
new file mode 100644
index 0000000..b0f44e3
--- /dev/null
+++ b/mysql/commands/ddl-commands.md
@@ -0,0 +1,82 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. CREATE 
+CREATE command is used to create a table, schema or an index.
+### Syntax:
+```sql
+         CREATE TABLE table_name (
+                column1 datatype,
+                column2 datatype,
+                ....);
+``` 
+### Example:
+```sql
+        CREATE TABLE CUSTOMERS(
+            InsuranceID INT,
+            Name VARCHAR(50),
+            DOB  DATE, 
+            NIN INT, 
+            Location VARCHAR(255)
+        );
+```
+## 2. ALTER
+ ALTER command is used to add, modify or delete columns or constraints from the database table.
+        
+###    Syntax: 
+```sql 
+ALTER TABLE Table_name ADD column_name datatype;
+```
+###    Example:
+```sql
+ ALTER TABLE CUSTOMERS ADD email_id VARCHAR(50);
+```
+## 3. TRUNCATE
+ TRUNCATE command is used to delete the data present in the table but this will not delete the table.
+###    Syntax: 
+```sql
+TRUNCATE table table_name;
+```
+### Example: 
+```sql
+TRUNCATE table CUSTOMERS;
+```
+## 4. DROP
+DROP command is used to delete the table along with its data.
+
+###    Syntax: 
+```sql 
+DROP TABLE table_name;
+```
+### Example: 
+```sql 
+DROP TABLE CUSTOMERS;
+```
+## 5. RENAME 
+RENAME command is used to rename the table name.
+
+###    Syntax:  
+```sql
+RENAME TABLE table_name1 to new_table_name1; 
+```
+### Example: 
+```sql
+RENAME TABLE CUSTOMERS to CUSTOMERINFO;
+```
+## 6. COMMENT
+
+###  Single-Line Comments: 
+Statements starting with `--` are treated as single line comments.
+ ###   Example:
+
+ ```sql
+  --Line1;
+  ```
+
+ ###   Multi-Line comments: 
+
+ Statements enclosed in `/**/` are treated as Multi-line comments
+
+ ```sql
+    /* Line1,
+    Line2 */
+ ````
\ No newline at end of file
diff --git a/mysql/commands/dml-commands.md b/mysql/commands/dml-commands.md
new file mode 100644
index 0000000..e1cd3cd
--- /dev/null
+++ b/mysql/commands/dml-commands.md
@@ -0,0 +1,54 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. INSERT  
+INSERT Statement  is used to insert new records into the database table.
+### Syntax:
+```sql
+INSERT INTO table_name (column1, column2, column3, ...) VALUES (value1, value2, value3, ...);
+```
+
+ Note: Column names are optional.
+
+ ###   Example: 
+ Both the below ways are correct.
+```sql
+     
+INSERT INTO CUSTOMERS (InsuranceID, Name, DOB, NIN, Location,email_id) VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com');
+INSERT INTO CUSTOMERS VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com'); 
+```
+## 2. SELECT 
+Select statement is used to select data from database tables.
+
+###   Syntax:
+```sql
+SELECT column1, column2, ...
+FROM table_name
+[where condition]; 
+```
+### Example: 
+```sql
+SELECT * FROM CUSTOMERS; 
+```    
+## 3. UPDATE 
+UPDATE statement is used to modify the existing values of records present in the database table.
+   ### Syntax:
+```sql
+ UPDATE table_name
+SET column1 = value1, column2 = value2, ...
+WHERE condition; 
+```
+ ###   Example: 
+ ```sql
+ UPDATE CUSTOMERS SET email_id = 'mango.lo@xyz.com' WHERE InsuranceID='123';
+```
+## 4. DELETE 
+DELETE statement is used to delete the existing records present in the database table.
+
+###    Syntax: 
+```sql 
+DELETE FROM table_name where condition;
+```
+###    Example: 
+```sql
+DELETE FROM CUSTOMERS where InsuranceID='123';
+```
\ No newline at end of file
diff --git a/mysql/commands/tcl-commands.md b/mysql/commands/tcl-commands.md
new file mode 100644
index 0000000..12f166b
--- /dev/null
+++ b/mysql/commands/tcl-commands.md
@@ -0,0 +1,27 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+## 1. COMMIT
+COMMIT statement is used to save all the changes made to the database in the current transaction.
+### Syntax: 
+```sql
+COMMIT;
+```
+
+## 2. ROLLBACK  
+ROLLBACK statement is used to revert the changes made to a database table in the current transaction.
+### Syntax:
+```sql
+ROLLBACK;
+```
+## 3. SAVEPOINT
+SAVEPOINT statement is used to define a identifier which points to the named transaction savepoint.
+### Syntax:
+```sql
+SAVEPOINT savepoint_name;
+ROLLBACK TO savepoint_name; 
+```
+How to delete a savepoint:
+```sql
+RELEASE SAVEPOINT savepoint_name;
+```
+
diff --git a/mysql/constraints/constraints.md b/mysql/constraints/constraints.md
new file mode 100644
index 0000000..d673708
--- /dev/null
+++ b/mysql/constraints/constraints.md
@@ -0,0 +1,21 @@
+
+Constraint is used to define rules on what values are inserted in the table. Constraint plays an important role in ensuring the accuracy and integrity of the data inserted into a table.
+
+Commonly used constraints are as follows:
+## 1. NOT NULL 
+If a column is specified as NOT NULL while defining the table, then the column must not have NULL values while inserting data into the table.
+## 2. UNIQUE
+ If a column is specified as UNIQUE while defining the table, then the column must have different values for all the rows present in the table.
+## 3. PRIMARY KEY 
+Primary key is a column or set of columns which is used to uniquely identify a row in the database table. In the below customer table, You can either choose InsuranceID or National Insurance Number(NIN) as primary keys, but Insurance ID is preferable as NIN can be considered as personal information.
+#### Table 1: 
+    customer(InsuranceID, Name, DOB, NIN, Location)
+#### Table 2: 
+    patient(HospitalID, Name, DOB, InsuranceID)
+
+## 4. FOREIGN KEY 
+Foreign key is a column in one table which references a primary key in other table. In the above two tables, InsuranceID of patient table can be foreign key which can be referenced to InsuranceID in customer Table.
+## 5. CHECK 
+checks for a specific condition while inserting data into the table. 
+## 6. DEFAULT 
+If a value is not specified for a column whose default value is mentioned while creating the table, then the default value is inserted while inserting data.
diff --git a/mysql/index.json b/mysql/index.json
new file mode 100644
index 0000000..24d520e
--- /dev/null
+++ b/mysql/index.json
@@ -0,0 +1,131 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title" : "About MySQL",
+                "path" : "about-mysql"
+            },
+            {
+                "title" : "Installation on Linux and Windows",
+                "path" : "installation"
+
+            }
+        ]
+    },
+    {
+        "title": "MySQL Data Types",
+        "path": "mysql-data-types",
+        "links": [
+            {
+                "title": "Numeric Data Types",
+                "path": "numeric-data-types"
+            },
+            {
+                "title": "Date and Time",
+                "path": "date-and-time"
+            },
+            {
+                "title": "String Data Types",
+                "path": "string-data-types"
+            }
+        ]
+    },
+    {
+        "title": "Keys",
+        "path": "keys",
+        "links": [
+            {
+                "title": "Keys",
+                "path": "keys"
+            }
+        ]
+    },
+    {
+        "title": "Constraints",
+        "path": "constraints",
+        "links": [
+            {
+                "title": "Constraints",
+                "path": "constraints"
+            }
+        ]
+    },
+    {
+        "title": "MySQL Commands",
+        "path": "commands",
+        "links": [
+            {
+                "title": "Commands",
+                "path": "commands"
+            },
+            {
+                "title": "DDL Commands",
+                "path": "ddl-commands"
+            },
+            {
+                "title": "DML Commands",
+                "path": "dml-commands"
+            },
+            {
+                "title": "DCL Commands",
+                "path": "dcl-commands"
+            },
+            {
+                "title": "TCL Commands",
+                "path": "tcl-commands"
+            }
+        ]
+    },
+    {
+        "title": "Joins",
+        "path": "joins",
+        "links": [
+            {
+                "title": "Joins",
+                "path": "joins"
+            }
+        ]
+    },
+    {
+        "title": "Indexes",
+        "path": "indexes",
+        "links": [
+            {
+                "title": "Indexes",
+                "path": "indexes"
+            }
+        ]
+    },
+    {
+        "title": "Triggers",
+        "path": "triggers",
+        "links": [
+            {
+                "title": "Triggers",
+                "path": "triggers"
+            }
+        ]
+    },
+    {
+        "title": "Views",
+        "path": "views",
+        "links": [
+            {
+                "title": "Views",
+                "path": "views"
+            }
+        ]
+    },
+    {
+        "title": "Stored Procedures",
+        "path": "stored-procedures",
+        "links": [
+            {
+                "title": "Stored Procedures",
+                "path": "stored-procedures"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/mysql/indexes/indexes.md b/mysql/indexes/indexes.md
new file mode 100644
index 0000000..fb2212c
--- /dev/null
+++ b/mysql/indexes/indexes.md
@@ -0,0 +1,18 @@
+In simpler terms, we can say INDEX is a pointer to some data in a table.INDEXES are very useful in attaining higher performance especially for large databases as it speeds up your search query execution.
+Unindexed tables take more time when compared to Indexed tables. For example, you can consider the synopsis of a book as an example for Indexes. You can easily search for a particular topic by referring it's page number in synopsis.  
+
+
+## 1. CREATE INDEX
+CREATE INDEX is used to create an index. INDEXes can be created on multiple columns(if required) of a table. Search query corresponding to INDEX column will be much faster than on other columns.
+
+### Syntax:
+```sql
+  CREATE INDEX index_name on table_name(column_name);
+```
+
+## 2. DROP INDEX
+DROP INDEX is used to delete an existing index
+### Syntax
+```sql
+DROP INDEX index_name ON table_name;
+```
\ No newline at end of file
diff --git a/mysql/introduction/about-mysql.md b/mysql/introduction/about-mysql.md
new file mode 100644
index 0000000..b03ac18
--- /dev/null
+++ b/mysql/introduction/about-mysql.md
@@ -0,0 +1,14 @@
+MySQL is a open-source, free and very popular relational database management system which is developed, distributed and supported by Oracle corporation. 
+
+## Key Features:
+*	Open-source relational database management systems.
+*	Reliable, very fast and easy to use database server.
+*	Works on client-server model.
+*	Allows users to customize based on their requirements.
+*	Highly Secure
+*	Scalable, Default size limit for a table is 4MB, but you can increase this to a theoretical limit of 8 million terabytes (TB)
+*	High Performance as it’s very fast, more reliable.
+*	High productivity as it uses stored procedures, triggers, views to write a highly productive code.
+*	Supports large databases efficiently.
+*	Supports many operating systems like Linux*,CentOS*, Solaris*,Ubuntu*,Windows*, MacOS*,FreeBSD* and others. 
+
diff --git a/mysql/introduction/installation.md b/mysql/introduction/installation.md
new file mode 100644
index 0000000..4879603
--- /dev/null
+++ b/mysql/introduction/installation.md
@@ -0,0 +1,29 @@
+# Installation on Linux
+
+## Using Yum
+
+* Download the required version from [MySQL Repository](https://dev.mysql.com/downloads/repo/yum/)
+* Install the downloaded release package with the below command, replace platform-and-version-specific-package-name with the name of the downloaded package:
+```shell
+shell> sudo rpm -Uvh platform-and-version-specific-package-name.rpm
+```
+* Install using below command
+```shell
+shell> sudo yum install mysql-community-server
+```
+* Start Mysql Server
+```shell
+shell> sudo service mysqld start
+```
+* Check status of the MySQL server
+```shell
+shell> sudo service mysqld status
+```
+# Installation on Windows
+
+* Installation on windows is much easier using [MySQL Installer](http://dev.mysql.com/downloads/installer/)
+* If you want to use online installation version, download mysql-installer-web-community.exe else mysql-installer-community.exe.
+* Run the above executable file and the setup will walk you through the trivial process.
+* select the Setup type based on your requirement among Developer default or server only or Client only or Full or Custom. 
+* Configure the MySQL Server by providing port number, password etc.
+* Installation will take few minutes to complete.
diff --git a/mysql/joins/joins.md b/mysql/joins/joins.md
new file mode 100644
index 0000000..8a26ed4
--- /dev/null
+++ b/mysql/joins/joins.md
@@ -0,0 +1,90 @@
+
+Joins are used along with SELECT statement when ever there is a need to retrieve data from multiple tables.
+
+Commonly used Joins in MySQL are as follows:
+
+1. INNER JOIN
+2. LEFT JOIN
+3. RIGHT JOIN
+4. CROSS JOIN
+
+Let us consider the below tables for our understanding:
+
+
+TABLE1: 
+
+
+|A|Z|
+|---|---|
+|a|1|
+|b|4|
+|d|5|
+
+
+TABLE2:                    
+
+|A|X|
+|---|---|
+|a|10|
+|e|17|
+|d|30|
+
+## 1. INNER JOIN
+INNER JOIN combines the values from both the tables based on matching criteria.
+
+Below is a basic example of how inner join works:
+```sql
+SELECT * FROM TABLE1 INNER JOIN TABLE2 where TABLE1.A=TABLE2.A;
+```
+
+#### Result:
+
+|A|Z|A|X|
+|--|--|--|--|
+|a|1|a|10|
+|d|5|d|30|
+
+## 2. LEFT JOIN
+LEFT JOIN returns all the values from the Left hand table of the condition and returns only the matching values from the second table. The unmatched row values of right hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 LEFT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|b|4|null|null|
+
+## 3. RIGHT JOIN
+RIGHT JOIN returns all the values from the right hand table of the condition and returns only the matching values from the left hand table. The unmatched row values of left hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 RIGHT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|null|null|e|17|
+
+## 4. CROSS JOIN
+CROSS JOIN matches each row of the first table with every row of the second table.
+```sql
+SELECT A,Z,X from TABLE1 CROSS JOIN TABLE2;
+```
+#### Result:
+
+|A|Z|X|
+|---|---|---|
+|a|1|10|
+|a|1|17|
+|a|1|30|
+|b|4|10|
+|b|4|17|
+|b|4|30|
+|d|5|10|
+|d|5|17|
+|d|5|30|
diff --git a/mysql/keys/keys.md b/mysql/keys/keys.md
new file mode 100644
index 0000000..2e656f1
--- /dev/null
+++ b/mysql/keys/keys.md
@@ -0,0 +1,23 @@
+let's consider the below two tables to understand the different keys present in SQL. 
+
+#### Table 1: 
+customer(InsuranceID, Name, DOB, NIN, Location)
+
+#### Table 2: 
+patient(HospitalID, Name, DOB, InsuranceID)
+
+## 1. Primary key 
+Primary key is a column or set of columns which is used to uniquely identify a row in the database table. In the above customer table, You can either choose InsuranceID or National Insurance Number(NIN) as primary keys, but Insurance ID is preferable as NIN can be considered as personal information.
+## 2. Super key 
+Super key is a superset of Primary key with other columns. In the above customer table, InsuranceID+Name can be a Super key. 
+## 3. Foreign key 
+Foreign key is a column in one table which references a primary key in other table. In the above two tables, InsuranceID of patient table can be considered as a foreign key which can be pointed to InsuranceID in customer Table.
+## 4. Unique key 
+Unique key is a column or set of columns which is used to uniquely identify a row in the database table. 
+
+Unique key is similar to primary key but differs in  below cases.
+* Unique key accepts only one null value where as primary key can never be null.
+* There can be multiple unique keys in a table where as primary key should be only one.
+* Primary key creates a clustered index where as unique key creates a non-clustered index. 
+## 5. Candidate key: 
+Candidate key is a column or set of columns which are eligible to become a primary key. In the customer table, InsuranceID and NIN can be candidate keys. As InsuranceID is a primary key, NIN can be a candidate key.
diff --git a/mysql/mysql-data-types/date-and-time.md b/mysql/mysql-data-types/date-and-time.md
new file mode 100644
index 0000000..254cedd
--- /dev/null
+++ b/mysql/mysql-data-types/date-and-time.md
@@ -0,0 +1,11 @@
+
+| Data type | Format | Range |
+| ---------- | ------ | ----- |
+| DATE | 'YYYY-MM-DD' | '1000-01-01' to '9999-12-31' |
+| DATETIME[(f)] | 'YYYY-MM-DD hh:mm:ss[.fraction]' | '1000-01-01 00:00:00.000000' to '9999-12-31 23:59:59.999999' | 
+| TIMESTAMP[(f)] | 'YYYY-MM-DD hh:mm:ss[.fraction]' | '1970-01-01 00:00:01.000000' UTC to '2038-01-19 03:14:07.999999' UTC | 
+| TIME[(f)] | 'hh:mm:ss[.fraction]' | '-838:59:59.000000' to '838:59:59.000000' | 
+| YEAR[(2/4)] | 'YY'/'YYYY'(default) | Year represented in 2 or 4 digits|
+
+
+Note: f refers to fractional seconds precision
diff --git a/mysql/mysql-data-types/numeric-data-types.md b/mysql/mysql-data-types/numeric-data-types.md
new file mode 100644
index 0000000..f72ccfd
--- /dev/null
+++ b/mysql/mysql-data-types/numeric-data-types.md
@@ -0,0 +1,16 @@
+## 1. Integer Family
+
+| Data Type |	Unsigned Min Value | Unsigned Max Value | Signed 	Min Value | Signed	Max Value |	
+| --------- | -------------------- | ------------------ | ----------------- | ----------------- |
+| INT	|0	| 4294967295 |	-2147483648 |	2147483647 |
+| SMALLINT  | 0	| 65535	| -32768 |	32767|
+| MEDIUMINT	| 0	| 16777215 |	-8388608 |	8388607 |
+| BIGINT	| 0	| 18446744073709551615| 	-9223372036854775808 |	9223372036854775807 |
+| TINYINT	| 0	| 255 |	-128 |	127 |
+
+## 2. FLOAT(d)
+where d is number of decimals. If the d value is in between  0-24 then the data type becomes FLOAT() else if the d value is >24 and <53 then the  data type becomes DOUBLE()
+## 3. DECIMAL(l,d)
+where l is number of digits displayed before the decimal point and d is number of digits after the decimal point. Default value is DECIMAL(10,2).
+
+
diff --git a/mysql/mysql-data-types/string-data-types.md b/mysql/mysql-data-types/string-data-types.md
new file mode 100644
index 0000000..80511e8
--- /dev/null
+++ b/mysql/mysql-data-types/string-data-types.md
@@ -0,0 +1,18 @@
+
+## 1. CHAR(l)
+CHAR is a fixed length string data type. Range can be from 0 to 255. Character values are stored with the specified length(l) and they are right padded with spaces.
+## 2. VARCHAR(l)
+VARCHAR is a variable length string data type. Range can be from 0 to 65,535 but 65,535 is the maximum row length which is shared among all columns. 
+## 3. BINARY and VARBINARY 
+BINARY and VARBINARY are very similar to CHAR and VARCHAR except that the data is stored as bytes where as the data is stored as characters for CHAR and VARCHAR. BINARY values are right padded with spaces. 
+
+## 4. BLOB and TEXT 
+BLOB values are treated as byte strings where as text values are treated as character strings.
+
+| Data Type | Maximum Size Limit |
+| --------- | ------------------ |
+| BLOB, TEXT | 65,535 |
+| TINYBLOB, TINYTEXT | 255 |
+| MEDIUMBLOB, MEDIUMTEXT | 16,777,215 |
+| LONGBLOB, LONG TEXT | 4,294,967,295 |
+
diff --git a/mysql/stored-procedures/stored-procedures.md b/mysql/stored-procedures/stored-procedures.md
new file mode 100644
index 0000000..34f897c
--- /dev/null
+++ b/mysql/stored-procedures/stored-procedures.md
@@ -0,0 +1,38 @@
+
+Stored Procedure is a sub-program/function which consists of a set of statements stored in the database server. Stored procedures usually consists of a generic code which can be reused at multiple places. We can also pass parameters while calling the stored procedures.
+
+#### Syntax:
+```sql
+CREATE PROCEDURE sp_name(p1 datatype)
+BEGIN
+/*Stored procedure code*/
+END;
+```
+## How to call Stored procedure
+```sql
+CALL sp_name;
+```
+
+## How to delete stored procedure
+```sql
+DROP PROCEDURE sp_name;
+```
+
+#### Example:
+Consider you are writing a procedure to retrieve all the order details of a particular Customer.
+```sql
+DELIMITER $$
+CREATE PROCEDURE ORDER_SUMMARY(IN customerid Varchar(30) )
+BEGIN
+SELECT * from ORDERS where CUSTOMERID=customerid;
+END$$
+DELIMITER ;
+```
+## How to call Stored Procedure
+```sql
+CALL ORDER_SUMMARY('C10');
+```
+#### Result:
+|ORDERID|CUSTOMERID|ITEM|BILLAMOUNT|
+|---|---|---|---|
+|123|C10|Mango|5.00|
diff --git a/mysql/triggers/triggers.md b/mysql/triggers/triggers.md
new file mode 100644
index 0000000..d38d810
--- /dev/null
+++ b/mysql/triggers/triggers.md
@@ -0,0 +1,44 @@
+
+Trigger is a set of actions which gets executed automatically when a specific event occurs like INSERT, UPDATE or DELETE.
+
+## How to create a TRIGGER?
+
+#### Syntax:
+```sql
+CREATE TRIGGER trigger_name trigger_time trigger_event
+    ON tbl_name FOR EACH ROW [trigger_order] trigger_body
+/* where
+trigger_time: { BEFORE | AFTER }
+trigger_event: { INSERT | UPDATE | DELETE }
+trigger_order: { FOLLOWS | PRECEDES } */
+```
+#### Example:
+In the below example, we will see ORDER_HISTORY table automaticlaly gets updated after a new record is inserted into ORDERS table. This is an example of AFTER INSERT, similarly we can use triggers for INSERT/UPDATE/DELETE events and based on the requirement choose whether the update on the second table should happen BEFORE or AFTER.
+
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+--creating trigger
+CREATE TRIGGER ORD_HIS AFTER INSERT ON ORDERS  FOR EACH ROW INSERT INTO ORDER_HISTORY values( new.ORDERID,new.ITEM,new.BILL_AMOUNT,NOW());
+
+--Now insert values into ORDERS table:
+INSERT INTO ORDERS VALUES(123,'C10','MANGO',5);
+```
+Check the ORDER_HISTORY table which will yield below results:
+
+|ORDERID|ITEM|WALLETAMOUNT|DATE_OF_ORDER|
+|--- |---|---|---|
+|123|Mango|5.00|2019-10-01|
+
+## How to delete a TRIGGER?
+
+#### Syntax:
+```sql
+ DROP TRIGGER trigger_name;
+```
+
+In the above example, if we want to delete the trigger we created,
+```sql
+DROP TRIGGER  ORD_HIS;
+```
diff --git a/mysql/views/views.md b/mysql/views/views.md
new file mode 100644
index 0000000..6e45bc2
--- /dev/null
+++ b/mysql/views/views.md
@@ -0,0 +1,36 @@
+
+View is a query which is stored in the database to reuse later. Views are nothing but a pseudo table. Views can contain a complex query which retrieves data from multiple tables. So, instead of writing a complex query everytime, you can simply create a view and call it when ever required.
+
+#### Syntax:
+```sql
+Creating a View:
+CREATE VIEW View_name AS 
+Query;
+```
+
+#### How to call view
+```sql
+SELECT * FROM View_name;
+```
+#### Altering a View
+```sql
+ALTER View View_name AS 
+Query;
+```
+#### Deleting a View
+```sql
+DROP VIEW View_name;
+```
+#### Example:
+Let us consider the below two tables:
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+-- creating view
+CREATE VIEW ORD_VIEW AS SELECT O.ORDERID,O.CUSTOMERID,H.ITEM,H.DATE_OF_ORDER from ORDERS O INNER JOIN ORDER_HISTORY H; 
+
+-- to display the view
+select * from ORD_VIEW;
+```
+In this way, we can also restrict the visibility of columns of the table to the users.
\ No newline at end of file
diff --git a/oracle/commands/commands.md b/oracle/commands/commands.md
new file mode 100644
index 0000000..a462304
--- /dev/null
+++ b/oracle/commands/commands.md
@@ -0,0 +1,25 @@
+This section helps you  how to write commands in Oracle. Let us categorize these commands into 4 categories.
+## 1. Data Definition Language(DDL) commands
+These commands are used to define the database structures. 
+* CREATE
+* ALTER
+* TRUNCATE
+* DROP
+* RENAME
+* COMMENT
+
+## 2. Data Manipulation Language(DML) commands 
+These commands are used to manipulate the data present in the database tables.
+* INSERT
+* SELECT
+* UPDATE
+* DELETE
+## 3. Data control language(DCL) commands 
+These commands are used to define the authorizations of the database like giving right permissions to the  users.
+* GRANT
+* REVOKE
+## 4. Transaction control Language(TCL) commands
+ These commands are used to manage the changes made to the data by DML commands. 
+ * COMMIT
+ * ROLLBACK
+ * SAVEPOINT
diff --git a/oracle/commands/dcl-commands.md b/oracle/commands/dcl-commands.md
new file mode 100644
index 0000000..1c66c46
--- /dev/null
+++ b/oracle/commands/dcl-commands.md
@@ -0,0 +1,53 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+# 1. GRANT
+GRANT statement is used to provide access privileges to users to access the database.
+
+### Syntax:
+```sql
+GRANT privileges_type ON object_type TO user;
+```
+
+**Note:** Privileges_type can be SELECT, INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, TRIGGER, CREATE, ALL. You can also specify combination of these privileges in a statement.
+object_type is table/function/procedure.
+
+
+
+### Grant global privileges to user
+
+```sql
+GRANT ALL ON *.* TO 'username'@'hostname';
+GRANT SELECT, INSERT ON *.* TO 'username'@'hostname';
+
+```
+
+### Grant database privileges to a user:
+```sql
+GRANT ALL ON dbname.* TO 'username'@'hostname';
+GRANT SELECT, INSERT ON dbname.* TO 'username'@'hostname';
+```
+
+
+### Grant table privileges to a user
+```sql
+GRANT ALL ON dbname.tblname TO 'username'@'hostname';
+GRANT SELECT, INSERT ON dbname.tblname TO 'username'@'hostname'';
+```
+### To Specify server to permit only encrypted connections
+```sql
+GRANT ALL PRIVILEGES ON test.* TO 'root'@'localhost' REQUIRE SSL;
+```
+
+## 2. REVOKE
+ REVOKE statement is used to withdraw the access priviliges given to a user by GRANT statement.
+
+#### Syntax:
+
+```sql
+REVOKE privileges ON object FROM user;
+```
+
+#### Example:
+```sql
+ REVOKE DELETE, UPDATE ON ORDERS FROM customer1;
+ ```
diff --git a/oracle/commands/ddl-commands.md b/oracle/commands/ddl-commands.md
new file mode 100644
index 0000000..b0f44e3
--- /dev/null
+++ b/oracle/commands/ddl-commands.md
@@ -0,0 +1,82 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. CREATE 
+CREATE command is used to create a table, schema or an index.
+### Syntax:
+```sql
+         CREATE TABLE table_name (
+                column1 datatype,
+                column2 datatype,
+                ....);
+``` 
+### Example:
+```sql
+        CREATE TABLE CUSTOMERS(
+            InsuranceID INT,
+            Name VARCHAR(50),
+            DOB  DATE, 
+            NIN INT, 
+            Location VARCHAR(255)
+        );
+```
+## 2. ALTER
+ ALTER command is used to add, modify or delete columns or constraints from the database table.
+        
+###    Syntax: 
+```sql 
+ALTER TABLE Table_name ADD column_name datatype;
+```
+###    Example:
+```sql
+ ALTER TABLE CUSTOMERS ADD email_id VARCHAR(50);
+```
+## 3. TRUNCATE
+ TRUNCATE command is used to delete the data present in the table but this will not delete the table.
+###    Syntax: 
+```sql
+TRUNCATE table table_name;
+```
+### Example: 
+```sql
+TRUNCATE table CUSTOMERS;
+```
+## 4. DROP
+DROP command is used to delete the table along with its data.
+
+###    Syntax: 
+```sql 
+DROP TABLE table_name;
+```
+### Example: 
+```sql 
+DROP TABLE CUSTOMERS;
+```
+## 5. RENAME 
+RENAME command is used to rename the table name.
+
+###    Syntax:  
+```sql
+RENAME TABLE table_name1 to new_table_name1; 
+```
+### Example: 
+```sql
+RENAME TABLE CUSTOMERS to CUSTOMERINFO;
+```
+## 6. COMMENT
+
+###  Single-Line Comments: 
+Statements starting with `--` are treated as single line comments.
+ ###   Example:
+
+ ```sql
+  --Line1;
+  ```
+
+ ###   Multi-Line comments: 
+
+ Statements enclosed in `/**/` are treated as Multi-line comments
+
+ ```sql
+    /* Line1,
+    Line2 */
+ ````
\ No newline at end of file
diff --git a/oracle/commands/dml-commands.md b/oracle/commands/dml-commands.md
new file mode 100644
index 0000000..86f7dbc
--- /dev/null
+++ b/oracle/commands/dml-commands.md
@@ -0,0 +1,54 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. INSERT  
+INSERT Statement  is used to insert new records into the database table.
+#### Syntax:
+```sql
+INSERT INTO table_name (column1, column2, column3, ...) VALUES (value1, value2, value3, ...);
+```
+
+ Note: Column names are optional.
+
+ ####   Example: 
+ Both the below ways are correct.
+```sql
+     
+INSERT INTO CUSTOMERS (InsuranceID, Name, DOB, NIN, Location,email_id) VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com');
+INSERT INTO CUSTOMERS VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com'); 
+```
+## 2. SELECT 
+Select statement is used to select data from database tables.
+
+####   Syntax:
+```sql
+SELECT column1, column2, ...
+FROM table_name
+[where condition]; 
+```
+#### Example: 
+```sql
+SELECT * FROM CUSTOMERS; 
+```    
+## 3. UPDATE 
+UPDATE statement is used to modify the existing values of records present in the database table.
+   #### Syntax:
+```sql
+ UPDATE table_name
+SET column1 = value1, column2 = value2, ...
+WHERE condition; 
+```
+ ####   Example: 
+ ```sql
+ UPDATE CUSTOMERS SET email_id = 'mango.lo@xyz.com' WHERE InsuranceID='123';
+```
+## 4. DELETE 
+DELETE statement is used to delete the existing records present in the database table.
+
+####    Syntax: 
+```sql 
+DELETE FROM table_name where condition;
+```
+####    Example: 
+```sql
+DELETE FROM CUSTOMERS where InsuranceID='123';
+```
\ No newline at end of file
diff --git a/oracle/commands/tcl-commands.md b/oracle/commands/tcl-commands.md
new file mode 100644
index 0000000..449797b
--- /dev/null
+++ b/oracle/commands/tcl-commands.md
@@ -0,0 +1,27 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+## 1. COMMIT
+COMMIT statement is used to save all the changes made to the database in the current transaction.
+#### Syntax: 
+```sql
+COMMIT;
+```
+
+## 2. ROLLBACK  
+ROLLBACK statement is used to revert the changes made to a database table in the current transaction.
+#### Syntax:
+```sql
+ROLLBACK;
+```
+## 3. SAVEPOINT
+SAVEPOINT statement is used to define a identifier which points to the named transaction savepoint.
+#### Syntax:
+```sql
+SAVEPOINT savepoint_name;
+ROLLBACK TO savepoint_name; 
+```
+How to delete a savepoint:
+```sql
+RELEASE SAVEPOINT savepoint_name;
+```
+
diff --git a/oracle/constraints/constraints.md b/oracle/constraints/constraints.md
new file mode 100644
index 0000000..d673708
--- /dev/null
+++ b/oracle/constraints/constraints.md
@@ -0,0 +1,21 @@
+
+Constraint is used to define rules on what values are inserted in the table. Constraint plays an important role in ensuring the accuracy and integrity of the data inserted into a table.
+
+Commonly used constraints are as follows:
+## 1. NOT NULL 
+If a column is specified as NOT NULL while defining the table, then the column must not have NULL values while inserting data into the table.
+## 2. UNIQUE
+ If a column is specified as UNIQUE while defining the table, then the column must have different values for all the rows present in the table.
+## 3. PRIMARY KEY 
+Primary key is a column or set of columns which is used to uniquely identify a row in the database table. In the below customer table, You can either choose InsuranceID or National Insurance Number(NIN) as primary keys, but Insurance ID is preferable as NIN can be considered as personal information.
+#### Table 1: 
+    customer(InsuranceID, Name, DOB, NIN, Location)
+#### Table 2: 
+    patient(HospitalID, Name, DOB, InsuranceID)
+
+## 4. FOREIGN KEY 
+Foreign key is a column in one table which references a primary key in other table. In the above two tables, InsuranceID of patient table can be foreign key which can be referenced to InsuranceID in customer Table.
+## 5. CHECK 
+checks for a specific condition while inserting data into the table. 
+## 6. DEFAULT 
+If a value is not specified for a column whose default value is mentioned while creating the table, then the default value is inserted while inserting data.
diff --git a/oracle/data-types/date-and-time.md b/oracle/data-types/date-and-time.md
new file mode 100644
index 0000000..254cedd
--- /dev/null
+++ b/oracle/data-types/date-and-time.md
@@ -0,0 +1,11 @@
+
+| Data type | Format | Range |
+| ---------- | ------ | ----- |
+| DATE | 'YYYY-MM-DD' | '1000-01-01' to '9999-12-31' |
+| DATETIME[(f)] | 'YYYY-MM-DD hh:mm:ss[.fraction]' | '1000-01-01 00:00:00.000000' to '9999-12-31 23:59:59.999999' | 
+| TIMESTAMP[(f)] | 'YYYY-MM-DD hh:mm:ss[.fraction]' | '1970-01-01 00:00:01.000000' UTC to '2038-01-19 03:14:07.999999' UTC | 
+| TIME[(f)] | 'hh:mm:ss[.fraction]' | '-838:59:59.000000' to '838:59:59.000000' | 
+| YEAR[(2/4)] | 'YY'/'YYYY'(default) | Year represented in 2 or 4 digits|
+
+
+Note: f refers to fractional seconds precision
diff --git a/oracle/data-types/numeric-data-types.md b/oracle/data-types/numeric-data-types.md
new file mode 100644
index 0000000..f72ccfd
--- /dev/null
+++ b/oracle/data-types/numeric-data-types.md
@@ -0,0 +1,16 @@
+## 1. Integer Family
+
+| Data Type |	Unsigned Min Value | Unsigned Max Value | Signed 	Min Value | Signed	Max Value |	
+| --------- | -------------------- | ------------------ | ----------------- | ----------------- |
+| INT	|0	| 4294967295 |	-2147483648 |	2147483647 |
+| SMALLINT  | 0	| 65535	| -32768 |	32767|
+| MEDIUMINT	| 0	| 16777215 |	-8388608 |	8388607 |
+| BIGINT	| 0	| 18446744073709551615| 	-9223372036854775808 |	9223372036854775807 |
+| TINYINT	| 0	| 255 |	-128 |	127 |
+
+## 2. FLOAT(d)
+where d is number of decimals. If the d value is in between  0-24 then the data type becomes FLOAT() else if the d value is >24 and <53 then the  data type becomes DOUBLE()
+## 3. DECIMAL(l,d)
+where l is number of digits displayed before the decimal point and d is number of digits after the decimal point. Default value is DECIMAL(10,2).
+
+
diff --git a/oracle/data-types/string-data-types.md b/oracle/data-types/string-data-types.md
new file mode 100644
index 0000000..80511e8
--- /dev/null
+++ b/oracle/data-types/string-data-types.md
@@ -0,0 +1,18 @@
+
+## 1. CHAR(l)
+CHAR is a fixed length string data type. Range can be from 0 to 255. Character values are stored with the specified length(l) and they are right padded with spaces.
+## 2. VARCHAR(l)
+VARCHAR is a variable length string data type. Range can be from 0 to 65,535 but 65,535 is the maximum row length which is shared among all columns. 
+## 3. BINARY and VARBINARY 
+BINARY and VARBINARY are very similar to CHAR and VARCHAR except that the data is stored as bytes where as the data is stored as characters for CHAR and VARCHAR. BINARY values are right padded with spaces. 
+
+## 4. BLOB and TEXT 
+BLOB values are treated as byte strings where as text values are treated as character strings.
+
+| Data Type | Maximum Size Limit |
+| --------- | ------------------ |
+| BLOB, TEXT | 65,535 |
+| TINYBLOB, TINYTEXT | 255 |
+| MEDIUMBLOB, MEDIUMTEXT | 16,777,215 |
+| LONGBLOB, LONG TEXT | 4,294,967,295 |
+
diff --git a/oracle/index.json b/oracle/index.json
new file mode 100644
index 0000000..d8018e4
--- /dev/null
+++ b/oracle/index.json
@@ -0,0 +1,131 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title" : "Introduction",
+                "path" : "introduction"
+            },
+            {
+                "title" : "Installation on Linux and Windows",
+                "path" : "installation"
+
+            }
+        ]
+    },
+    {
+        "title": "Data Types",
+        "path": "data-types",
+        "links": [
+            {
+                "title": "Numeric Data Types",
+                "path": "numeric-data-types"
+            },
+            {
+                "title": "Date and Time",
+                "path": "date-and-time"
+            },
+            {
+                "title": "String Data Types",
+                "path": "string-data-types"
+            }
+        ]
+    },
+    {
+        "title": "Keys",
+        "path": "keys",
+        "links": [
+            {
+                "title": "Keys",
+                "path": "keys"
+            }
+        ]
+    },
+    {
+        "title": "Constraints",
+        "path": "constraints",
+        "links": [
+            {
+                "title": "Constraints",
+                "path": "constraints"
+            }
+        ]
+    },
+    {
+        "title": "Commands",
+        "path": "commands",
+        "links": [
+            {
+                "title": "Commands",
+                "path": "commands"
+            },
+            {
+                "title": "DDL Commands",
+                "path": "ddl-commands"
+            },
+            {
+                "title": "DML Commands",
+                "path": "dml-commands"
+            },
+            {
+                "title": "DCL Commands",
+                "path": "dcl-commands"
+            },
+            {
+                "title": "TCL Commands",
+                "path": "tcl-commands"
+            }
+        ]
+    },
+    {
+        "title": "Joins",
+        "path": "joins",
+        "links": [
+            {
+                "title": "Joins",
+                "path": "joins"
+            }
+        ]
+    },
+    {
+        "title": "Indexes",
+        "path": "indexes",
+        "links": [
+            {
+                "title": "Indexes",
+                "path": "indexes"
+            }
+        ]
+    },
+    {
+        "title": "Triggers",
+        "path": "triggers",
+        "links": [
+            {
+                "title": "Triggers",
+                "path": "triggers"
+            }
+        ]
+    },
+    {
+        "title": "Views",
+        "path": "views",
+        "links": [
+            {
+                "title": "Views",
+                "path": "views"
+            }
+        ]
+    },
+    {
+        "title": "Stored Procedures",
+        "path": "stored-procedures",
+        "links": [
+            {
+                "title": "Stored Procedures",
+                "path": "stored-procedures"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/oracle/indexes/indexes.md b/oracle/indexes/indexes.md
new file mode 100644
index 0000000..cc76b9a
--- /dev/null
+++ b/oracle/indexes/indexes.md
@@ -0,0 +1,18 @@
+In simpler terms, we can say INDEX is a pointer to some data in a table.INDEXES are very useful in attaining higher performance especially for large databases as it speeds up your search query execution.
+Unindexed tables take more time when compared to Indexed tables. For example, you can consider the synopsis of a book as an example for Indexes. You can easily search for a particular topic by referring it's page number in synopsis.  
+
+
+## 1. CREATE INDEX
+CREATE INDEX is used to create an index. INDEXes can be created on multiple columns(if required) of a table. Search query corresponding to iNDEX column will be much faster than on other columns.
+
+### Syntax:
+```sql
+  CREATE INDEX index_name on table_name(column_name);
+```
+
+## 2. DROP INDEX
+DROP INDEX is used to delete an existing index
+### Syntax
+```sql
+DROP INDEX index_name ON table_name;
+```
\ No newline at end of file
diff --git a/oracle/introduction/installation.md b/oracle/introduction/installation.md
new file mode 100644
index 0000000..a85367c
--- /dev/null
+++ b/oracle/introduction/installation.md
@@ -0,0 +1,20 @@
+
+## Installation on Windows
+
+1. Download the required version and edition from [Oracle Downloads](https://www.oracle.com/database/technologies/oracle-database-software-downloads.html)
+2. Extract the zip contents and run setup.exe
+3. setup will walk you through the trivial process.
+
+
+## Installation on Linux
+1. Download the required version and edition from [Oracle Downloads](https://www.oracle.com/database/technologies/oracle-database-software-downloads.html)
+2. login as root and Unzip files
+    ```shell
+    unzip linux.x64_11gR2_database_1of2.zip
+    unzip linux.x64_11gR2_database_2of2.zip
+    ```
+3. Using `yum`:
+If you plan to use the "oracle-validated" package to perform all your required prerequisite setup, follow the instructions [here](http://public-yum.oracle.com) to setup the yum repository for OL, then perform the following command.
+    ```shell
+    yum install oracle-validated
+    ```
diff --git a/oracle/introduction/introduction.md b/oracle/introduction/introduction.md
new file mode 100644
index 0000000..667b6ca
--- /dev/null
+++ b/oracle/introduction/introduction.md
@@ -0,0 +1,20 @@
+Oracle is an efficient relational database system which is developed and marketed by Oracle Corporation. 
+Oracle is available in 5 different editions to support different development and deployment scenarios
+
+1. Enterprise Edition(EE)
+    * For medium to very large implementations.
+    * no limitation on server resources available to database.
+    * offers very good scalability and reliability in both clustered and single system configurations
+
+2. Standard Edition(SE)
+    * for small to medium sized implementations
+    * Includes Real application Clusters with a limitation of 2 sockets and 16 concurrent user threads.
+    * Upwardly compatible to EE
+3. Personal Edition (PE)
+    * for single user, single machine development and deployment
+    * Upwardly compatible to EE and SE2
+4. Express Edition (XE)
+    * Free entry level version for both windows and linux
+    * Limitations up to 2 CPUs, up to 2GB RAM and 12GB of user data.
+5. Oracle Lite
+    * Designed for mobile devices.
diff --git a/oracle/joins/joins.md b/oracle/joins/joins.md
new file mode 100644
index 0000000..aa2be9d
--- /dev/null
+++ b/oracle/joins/joins.md
@@ -0,0 +1,90 @@
+
+Joins are used along with SELECT statement when ever there is a need to retrieve data from multiple tables.
+
+Commonly used Joins in Oracle are as follows:
+
+1. INNER JOIN
+2. LEFT JOIN
+3. RIGHT JOIN
+4. CROSS JOIN
+
+Let us consider the below tables for our understanding:
+
+
+TABLE1: 
+
+
+|A|Z|
+|---|---|
+|a|1|
+|b|4|
+|d|5|
+
+
+TABLE2:                    
+
+|A|X|
+|---|---|
+|a|10|
+|e|17|
+|d|30|
+
+## 1. INNER JOIN
+INNER JOIN combines the values from both the tables based on matching criteria.
+
+Below is a basic example of how inner join works:
+```sql
+SELECT * FROM TABLE1 INNER JOIN TABLE2 where TABLE1.A=TABLE2.A;
+```
+
+#### Result:
+
+|A|Z|A|X|
+|--|--|--|--|
+|a|1|a|10|
+|d|5|d|30|
+
+## 2. LEFT JOIN
+LEFT JOIN returns all the values from the Left hand table of the condition and returns only the matching values from the second table. The unmatched row values of right hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 LEFT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|b|4|null|null|
+
+## 3. RIGHT JOIN
+RIGHT JOIN returns all the values from the right hand table of the condition and returns only the matching values from the left hand table. The unmatched row values of left hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 RIGHT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|null|null|e|17|
+
+## 4. CROSS JOIN
+CROSS JOIN matches each row of the first table with every row of the second table.
+```sql
+SELECT A,Z,X from TABLE1 CROSS JOIN TABLE2;
+```
+#### Result:
+
+|A|Z|X|
+|---|---|---|
+|a|1|10|
+|a|1|17|
+|a|1|30|
+|b|4|10|
+|b|4|17|
+|b|4|30|
+|d|5|10|
+|d|5|17|
+|d|5|30|
diff --git a/oracle/keys/keys.md b/oracle/keys/keys.md
new file mode 100644
index 0000000..5255162
--- /dev/null
+++ b/oracle/keys/keys.md
@@ -0,0 +1,23 @@
+let's consider the below two tables to understand the different keys. 
+
+#### Table 1: 
+customer(InsuranceID, Name, DOB, NIN, Location)
+
+#### Table 2: 
+patient(HospitalID, Name, DOB, InsuranceID)
+
+## 1. Primary key 
+Primary key is a column or set of columns which is used to uniquely identify a row in the database table. In the above customer table, You can either choose InsuranceID or National Insurance Number(NIN) as primary keys, but Insurance ID is preferable as NIN can be considered as personal information.
+## 2. Super key 
+Super key is a superset of Primary key with other columns. In the above customer table, InsuranceID+Name can be a Super key. 
+## 3. Foreign key 
+Foreign key is a column in one table which references a primary key in other table. In the above two tables, InsuranceID of patient table can be considered as a foreign key which can be pointed to InsuranceID in customer Table.
+## 4. Unique key 
+Unique key is a column or set of columns which is used to uniquely identify a row in the database table. 
+
+Unique key is similar to primary key but differs in  below cases.
+* Unique key accepts only one null value where as primary key can never be null.
+* There can be multiple unique keys in a table where as primary key should be only one.
+* Primary key creates a clustered index where as unique key creates a non-clustered index. 
+## 5. Candidate key: 
+Candidate key is a column or set of columns which are eligible to become a primary key. In the customer table, InsuranceID and NIN can be candidate keys. As InsuranceID is a primary key,NIN can be a candidate key.
diff --git a/oracle/stored-procedures/stored-procedures.md b/oracle/stored-procedures/stored-procedures.md
new file mode 100644
index 0000000..94f594b
--- /dev/null
+++ b/oracle/stored-procedures/stored-procedures.md
@@ -0,0 +1,38 @@
+
+Stored Procedure is a sub-program/function which consists of a set of statements stored in the database server. Stored procedures usually consists of a generic code which can reused at multiple places. We can also pass parameters while calling the stored procedures.
+
+#### Syntax:
+```sql
+CREATE PROCEDURE sp_name(p1 datatype)
+BEGIN
+/*Stored procedure code*/
+END;
+```
+## How to call Stored procedure
+```sql
+CALL sp_name;
+```
+
+## How to delete stored procedure
+```sql
+DROP PROCEDURE sp_name;
+```
+
+#### Example:
+Consider you are writing a procedure to retrieve all the order details of a particular Customer.
+```sql
+DELIMITER $$
+CREATE PROCEDURE ORDER_SUMMARY(IN customerid Varchar(30) )
+BEGIN
+SELECT * from ORDERS where CUSTOMERID=customerid;
+END$$
+DELIMITER ;
+```
+## How to call Stored Procedure
+```sql
+CALL ORDER_SUMMARY('C10');
+```
+#### Result:
+|ORDERID|CUSTOMERID|ITEM|BILLAMOUNT|
+|---|---|---|---|
+|123|C10|Mango|5.00|
diff --git a/oracle/triggers/triggers.md b/oracle/triggers/triggers.md
new file mode 100644
index 0000000..59172ce
--- /dev/null
+++ b/oracle/triggers/triggers.md
@@ -0,0 +1,44 @@
+
+Trigger is a set of actions which gets executed automatically when a specific event occurs like INSERT, UPDATE or DELETE.
+
+## HOW to CREATE a TRIGGER?
+
+#### Syntax:
+```sql
+CREATE TRIGGER trigger_name trigger_time trigger_event
+    ON tbl_name [FOR EACH ROW] [trigger_order] trigger_body
+/* where
+trigger_time: { BEFORE | AFTER }
+trigger_event: { INSERT | UPDATE | DELETE }
+trigger_order: { FOLLOWS | PRECEDES } */
+```
+#### Example:
+In the below example, we will see ORDER_HISTORY table automaticlaly gets updated after a new record is inserted into ORDERS table. This is an example of AFTER INSERT, similarly we can use triggers for INSERT/UPDATE/DELETE events and based on the requirement choose whether the update on the second table should happen BEFORE or AFTER.
+
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+--creating trigger
+CREATE TRIGGER ORD_HIS AFTER INSERT ON ORDERS  FOR EACH ROW INSERT INTO ORDER_HISTORY values( new.ORDERID,new.ITEM,new.BILL_AMOUNT,NOW());
+
+--Now insert values into ORDERS table:
+INSERT INTO ORDERS VALUES(123,'C10','MANGO',5);
+```
+Check the ORDER_HISTORY table which will yield below results:
+
+|ORDERID|ITEM|WALLETAMOUNT|DATE_OF_ORDER|
+|--- |---|---|---|
+|123|Mango|5.00|2019-10-01|
+
+## HOW to DELETE a TRIGGER?
+
+#### Syntax:
+```sql
+ DROP TRIGGER trigger_name;
+```
+
+In the above example, if we want to delete the trigger which you have created,
+```sql
+DROP TRIGGER  ORD_HIS;
+```
diff --git a/oracle/views/views.md b/oracle/views/views.md
new file mode 100644
index 0000000..6e45bc2
--- /dev/null
+++ b/oracle/views/views.md
@@ -0,0 +1,36 @@
+
+View is a query which is stored in the database to reuse later. Views are nothing but a pseudo table. Views can contain a complex query which retrieves data from multiple tables. So, instead of writing a complex query everytime, you can simply create a view and call it when ever required.
+
+#### Syntax:
+```sql
+Creating a View:
+CREATE VIEW View_name AS 
+Query;
+```
+
+#### How to call view
+```sql
+SELECT * FROM View_name;
+```
+#### Altering a View
+```sql
+ALTER View View_name AS 
+Query;
+```
+#### Deleting a View
+```sql
+DROP VIEW View_name;
+```
+#### Example:
+Let us consider the below two tables:
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+-- creating view
+CREATE VIEW ORD_VIEW AS SELECT O.ORDERID,O.CUSTOMERID,H.ITEM,H.DATE_OF_ORDER from ORDERS O INNER JOIN ORDER_HISTORY H; 
+
+-- to display the view
+select * from ORD_VIEW;
+```
+In this way, we can also restrict the visibility of columns of the table to the users.
\ No newline at end of file
diff --git a/postgresql/commands/commands.md b/postgresql/commands/commands.md
new file mode 100644
index 0000000..5883150
--- /dev/null
+++ b/postgresql/commands/commands.md
@@ -0,0 +1,25 @@
+This section helps you  how to write commands in PostgreSQL. Let us categorize these commands into 4 categories.
+## 1. Data Definition Language(DDL) commands
+These commands are used to define the database structures. 
+* CREATE
+* ALTER
+* TRUNCATE
+* DROP
+* RENAME
+* COMMENT
+
+## 2. Data Manipulation Language(DML) commands
+These commands are used to manipulate the data present in the database tables.
+* INSERT
+* SELECT
+* UPDATE
+* DELETE
+## 3. Data control language(DCL) commands
+These commands are used to define the authorizations of the database like giving right permissions to the  users.
+* GRANT
+* REVOKE
+## 4. Transaction control Language(TCL) commands
+ These commands are used to manage the changes made to the data by DML commands. 
+ * COMMIT
+ * ROLLBACK
+ * SAVEPOINT
diff --git a/postgresql/commands/dcl-commands.md b/postgresql/commands/dcl-commands.md
new file mode 100644
index 0000000..afb14f6
--- /dev/null
+++ b/postgresql/commands/dcl-commands.md
@@ -0,0 +1,62 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+# 1. GRANT
+GRANT statement is used to provide access privileges to users to access the database.
+
+## Syntax
+```sql
+GRANT privileges ON object TO user;
+```
+
+**Note:** Privileges can be SELECT, INSERT, UPDATE, DELETE, TRUNCATE, REFERENCES, TRIGGER, CREATE, ALL. You can also specify combination of these privileges in a statement.
+
+
+#### GRANT Connect to Database
+
+```sql
+GRANT CONNECT ON DATABASE database_name TO username;
+```
+
+#### GRANT Usage on Schema
+
+```sql
+GRANT USAGE ON SCHEMA database_name TO username;
+```
+
+#### Grant access to all tables in the database
+
+```sql
+GRANT ALL PRIVILEGES ON ALL TABLES IN SCHEMA schema_name TO username;
+GRANT ALL PRIVILEGES ON ALL SEQUENCES IN SCHEMA schema_name TO username;
+GRANT ALL PRIVILEGES ON DATABASE database_name TO username;
+```
+
+
+
+
+
+#### Grant permission to create database
+```sql
+ALTER USER username CREATEDB;
+```
+
+
+#### Grant superuser access to a user
+```sql
+ALTER USER myuser WITH SUPERUSER;
+```
+
+
+## 2. REVOKE
+ REVOKE statement is used to withdraw the access priviliges given to a user by GRANT statement.
+
+#### Syntax:
+
+```sql
+REVOKE privileges ON object FROM user;
+```
+
+#### Example
+```sql
+ REVOKE DELETE, UPDATE ON ORDERS FROM customer1;
+ ```
diff --git a/postgresql/commands/ddl-commands.md b/postgresql/commands/ddl-commands.md
new file mode 100644
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--- /dev/null
+++ b/postgresql/commands/ddl-commands.md
@@ -0,0 +1,79 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. CREATE
+CREATE command is used to create a table, schema or an index.
+#### Syntax:
+```sql
+         CREATE TABLE table_name (
+                column1 datatype,
+                column2 datatype,
+                ....);
+``` 
+#### Example:
+```sql
+        CREATE TABLE CUSTOMERS(
+            InsuranceID INT,
+            Name VARCHAR(50),
+            DOB  DATE, 
+            NIN INT, 
+            Location VARCHAR(255)
+        );
+```
+## 2. ALTER
+ ALTER command is used to add, modify or delete columns or constraints from the database table.
+        
+#### Syntax
+```sql 
+ALTER TABLE Table_name ADD column_name datatype;
+```
+#### Example
+```sql
+ ALTER TABLE CUSTOMERS ADD email_id VARCHAR(50);
+```
+## 3. TRUNCATE: 
+ TRUNCATE command is used to delete the data present in the table but this will not delete the table.
+#### Syntax
+```sql
+TRUNCATE table table_name;
+```
+#### Example
+```sql
+TRUNCATE table CUSTOMERS;
+```
+## 4. DROP
+DROP command is used to delete the table along with its data.
+
+#### Syntax
+```sql 
+DROP TABLE table_name;
+```
+#### Example
+```sql 
+DROP TABLE CUSTOMERS;
+```
+## 5. RENAME
+RENAME command is used to rename the table name.
+
+#### Syntax
+```sql
+ALTER TABLE table_name1 RENAME to new_table_name1; 
+```
+#### Example
+```sql
+ALTER TABLE CUSTOMERS RENAME to CUSTOMERINFO;
+```
+## 6. COMMENT
+
+####  Single-Line Comments: 
+Statements starting with -- are treated as single line comments.
+ #### Example:
+ ```sql
+  --Line1;
+ ```
+
+ #### Multi-Line comments: 
+ Statements enclosed in /**/ are treated as Multi-line comments
+ ```sql
+    /* Line1,
+    Line2 */
+ ```
\ No newline at end of file
diff --git a/postgresql/commands/dml-commands.md b/postgresql/commands/dml-commands.md
new file mode 100644
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--- /dev/null
+++ b/postgresql/commands/dml-commands.md
@@ -0,0 +1,54 @@
+In this sub section, let us learn the usage of below commands with examples.
+
+## 1. INSERT
+INSERT Statement  is used to insert new records into the database table.
+#### Syntax
+```sql
+INSERT INTO table_name (column1, column2, column3, ...) VALUES (value1, value2, value3, ...);
+```
+
+ Note: Column names are optional.
+
+ #### Example
+ Both the below ways are correct.
+```sql
+     
+INSERT INTO CUSTOMERS (InsuranceID, Name, DOB, NIN, Location,email_id) VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com');
+INSERT INTO CUSTOMERS VALUES ('123', 'Mango','2000-01-01','56789','LO','Mango@xyz.com'); 
+```
+## 2. SELECT
+Select statement is used to select data from database tables.
+
+####   Syntax:
+```sql
+SELECT column1, column2, ...
+FROM table_name; 
+```
+#### Example
+```sql
+SELECT * FROM CUSTOMERS; 
+```    
+## 3. UPDATE
+UPDATE statement is used to modify the existing values of records present in the database table.
+
+#### Syntax
+```sql
+UPDATE table_name
+SET column1 = value1, column2 = value2, ...
+WHERE condition; 
+```
+####   Example
+```sql
+UPDATE CUSTOMERS SET email_id = 'mango.lo@xyz.com' WHERE InsuranceID='123';
+```
+## 4. DELETE
+DELETE statement is used to delete the existing records present in the database table.
+
+#### Syntax
+```sql 
+DELETE FROM table_name where condition;
+```
+#### Example
+```sql
+DELETE FROM CUSTOMERS where InsuranceID='123';
+```
\ No newline at end of file
diff --git a/postgresql/commands/tcl-commands.md b/postgresql/commands/tcl-commands.md
new file mode 100644
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--- /dev/null
+++ b/postgresql/commands/tcl-commands.md
@@ -0,0 +1,33 @@
+In this sub section, let us learn the usage of below commands in detail.
+
+## 1. COMMIT
+COMMIT statement is used to save all the changes made to the database in the current transaction.
+
+#### Syntax
+```sql
+COMMIT;
+```
+
+## 2. ROLLBACK
+ROLLBACK statement is used to revert the changes made to a database table in the current transaction.
+
+#### Syntax:
+```sql
+ROLLBACK;
+```
+
+## 3. SAVEPOINT
+SAVEPOINT statement is used to define a identifier which points to the named transaction savepoint.
+
+#### Syntax:
+
+```sql
+SAVEPOINT savepoint_name;
+ROLLBACK TO savepoint_name; 
+```
+
+How to delete a savepoint:
+```sql
+RELEASE SAVEPOINT savepoint_name;
+```
+
diff --git a/postgresql/constraints/constraints.md b/postgresql/constraints/constraints.md
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--- /dev/null
+++ b/postgresql/constraints/constraints.md
@@ -0,0 +1,22 @@
+Constraint is used to define rules on what values are inserted in the table. Constraint plays an important role in ensuring the accuracy and integrity of the data inserted into a table.
+
+Commonly used constraints are as follows
+## 1. NOT NULL
+If a column is specified as NOT NULL while defining the table, then the column must not have NULL values while inserting data into the table.
+## 2. UNIQUE
+ If a column is specified as UNIQUE while defining the table, then the column must have different values for all the rows present in the table.
+## 3. PRIMARY KEY
+Primary key is a column or set of columns which is used to uniquely identify a row in the database table. In the below customer table, You can either choose InsuranceID or National Insurance Number(NIN) as primary keys, but Insurance ID is preferable as NIN can be considered as personal information.
+#### Table 1
+customer(InsuranceID, Name, DOB, NIN, Location)
+#### Table 2
+patient(HospitalID, Name, DOB, InsuranceID)
+
+## 4. FOREIGN KEY
+ Foreign key is a column in one table which references a primary key in other table. In the above two tables, InsuranceID of patient table is a foreign key which can be referenced to InsuranceID in customer Table.
+ 
+## 5. CHECK
+ checks for a specific condition while inserting data into the table. 
+
+## 6. DEFAULT
+If a value is not specified for a column whose default value is mentioned while creating the table, then the default value is inserted while inserting data.
diff --git a/postgresql/data-types/data-types.md b/postgresql/data-types/data-types.md
new file mode 100644
index 0000000..22ea93e
--- /dev/null
+++ b/postgresql/data-types/data-types.md
@@ -0,0 +1,13 @@
+
+
+|Category| Data Types|
+|---|---|
+|Primitives|Integer, Numeric, Character (text, varchar, char), Boolean, Binary|
+|Structured| Date/Time(timestamp/time with/without timezone, date, interval), Array, Range, UUID|
+|Document| JSON/JSONB, XML, Key-value (Hstore)|
+|Geometry|Point, Line, Circle, Polygon |
+|Customizations| Composite, Custom Types|
+|Misc| Money, Enum, Universally unique identifier (UUID), Classless Inter-Domain Routing (CIDR) blocks and MAC addresses, IPv4 and IPv6 addresses|
+
+#### Note
+In addition to the above data types, you can also create your own data types.
\ No newline at end of file
diff --git a/postgresql/index.json b/postgresql/index.json
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--- /dev/null
+++ b/postgresql/index.json
@@ -0,0 +1,102 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title": "About POSTGRES",
+                "path": "about-postgres"
+            },
+            {
+                "title": "Installation on Linux and Windows",
+                "path": "installation"
+            }
+        ]
+    },
+    {
+        "title": "Data Types",
+        "path": "data-types",
+        "links": [
+            {
+                "title": "Data Types",
+                "path": "data-types"
+            }
+        ]
+    },
+    {
+        "title": "Commands",
+        "path": "commands",
+        "links": [
+            {
+                "title": "Commands",
+                "path": "commands"
+            },
+            {
+                "title": "DDL Commands",
+                "path": "ddl-commands"
+            },
+            {
+                "title": "DML Commands",
+                "path": "dml-commands"
+            },
+            {
+                "title": "DCL Commands",
+                "path": "dcl-commands"
+            },
+            {
+                "title": "TCL Commands",
+                "path": "tcl-commands"
+            }
+        ]
+    },
+    {
+        "title": "Constraints",
+        "path": "constraints",
+        "links": [
+            {
+                "title": "Constraints",
+                "path": "constraints"
+            }
+        ]
+    },
+    {
+        "title": "Joins",
+        "path": "joins",
+        "links": [
+            {
+                "title": "Joins",
+                "path": "joins"
+            }
+        ]
+    },
+    {
+        "title": "Indexes",
+        "path": "indexes",
+        "links": [
+            {
+                "title": "Indexes",
+                "path": "indexes"
+            }
+        ]
+    },
+    {
+        "title": "Triggers",
+        "path": "triggers",
+        "links": [
+            {
+                "title": "Triggers",
+                "path": "triggers"
+            }
+        ]
+    },
+    {
+        "title": "Views",
+        "path": "views",
+        "links": [
+            {
+                "title": "Views",
+                "path": "views"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/postgresql/indexes/indexes.md b/postgresql/indexes/indexes.md
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+In simpler terms, we can say INDEX is a pointer to some data in a table. INDEXES are very useful in attaining higher performance especially for large databases as it speeds up your search query execution.
+Unindexed tables take more time when compared to Indexed tables. For example, you can consider the synopsis of a book as an example for Indexes. You can easily search for a particular topic by referring it's page number in synopsis.  
+
+
+## 1. CREATE INDEX
+Create Index is used to create an index. INDEXes can be created on multiple columns(if required) of a table. Search query corresponding to INDEX column will be much faster than on other columns.
+#### Syntax
+```sql
+CREATE INDEX index_name on table_name(column_name);
+```
+
+## 2. DROP INDEX
+Drop index is used to delete an index
+
+#### Syntax
+```sql
+DROP INDEX index_name;
+```
\ No newline at end of file
diff --git a/postgresql/introduction/about-postgres.md b/postgresql/introduction/about-postgres.md
new file mode 100644
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+++ b/postgresql/introduction/about-postgres.md
@@ -0,0 +1,9 @@
+PostgreSQL is a open source relational database system and is also knows as Postgres. 
+
+* Postgres is not only free and open-source but also it is highly extensible. 
+* Custom Data types and funtions from various programming languaues can be introduced and the good part is compiling entire database is not required.
+* ACID(Atomicity, Consistency, Isolation, Durability) compliant.
+* First DBMS which implemented Multi-version concurrency control (MVCC) feature.
+* It's the default database server for MacOS.
+* It supports all major operating systems like Linux, Windows, OpenBSD,FreeBSD etc. 
+ 
\ No newline at end of file
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diff --git a/postgresql/introduction/installation.md b/postgresql/introduction/installation.md
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+++ b/postgresql/introduction/installation.md
@@ -0,0 +1,43 @@
+## Installation on Linux
+
+1. Download the required version from [EnterpriseDB](https://www.enterprisedb.com/downloads/postgres-postgresql-downloads)
+2. Login as root and execute below commands
+    ```
+    [root@host]# chmod +x postgresql-10.10-1-linux-x64.run
+    [root@host]# ./postgresql-10.10-1-linux-x64.run
+    ```
+3. Provide location, port number, password of the username etc., to complete the installation. 
+
+## Installation on Windows
+
+1. Download the required version from [EnterpriseDB](https://www.enterprisedb.com/downloads/postgres-postgresql-downloads)
+![](images/postgres-installation-on-windows-1.png)
+2. Run the executable file and specify the installation directory.
+![](images/postgres-installation-on-windows-2.png)
+3. Select the components as per your requirement. Stack Builder is used to download additional components for future purpose.
+![](images/postgres-installation-on-windows-3.png)
+4. Provide the password.
+![](images/postgres-installation-on-windows-4.png)
+5. provide port number
+![](images/postgres-installation-on-windows-5.png)
+6. Setup is ready now to start the installation process.
+![](images/postgres-installation-on-windows-6.png)
+7. Installation takes few minutes to complete.
+![](images/postgres-installation-on-windows-7.png)
+8. Verify the installation.
+    * Open psql
+    * Provide servername, database name, port number, user name and password which you have provided during installation.
+    * Check the version using 
+        ```sql
+        select version();
+        ```
+        ![](images/postgres-installation-on-windows-8.png)
+
+## Connect to database
+
+1. open pgadmin 4
+2. provide password
+3. If server is not already available, create by right clicking on Servers and provide hostname, username and passsword.
+    ![](images/postgres-installation-on-windows-9.png)
+4. Open Querytool by right clicking on the database you created.
+    ![](images/postgres-installation-on-windows-10.png)
diff --git a/postgresql/joins/joins.md b/postgresql/joins/joins.md
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+++ b/postgresql/joins/joins.md
@@ -0,0 +1,88 @@
+
+Joins are used along with SELECT statement when ever there is a need to retrieve data from multiple tables.
+
+Commonly used Joins in Postgres are as follows:
+
+1. INNER JOIN
+2. LEFT JOIN
+3. RIGHT JOIN
+4. CROSS JOIN
+
+Let us consider the below tables for our understanding:
+
+TABLE1: 
+                   
+|A|Z|
+|---|---|
+|a|1|
+|b|4|
+|d|5|
+
+
+TABLE2:                    
+
+|A|X|
+|---|---|
+|a|10|
+|e|17|
+|d|30|
+
+## 1. INNER JOIN:
+INNER JOIN combines the values from both the tables based on matching criteria.
+
+Below is a basic example of how inner join works:
+```sql
+SELECT * FROM TABLE1 INNER JOIN TABLE2 where TABLE1.A=TABLE2.A;
+```
+
+#### Result:
+
+|A|Z|A|X|
+|--|--|--|--|
+|a|1|a|10|
+|d|5|d|30|
+
+## 2. LEFT JOIN:
+LEFT JOIN returns all the values from the Left hand table of the condition and returns only the matching values from the second table. The unmatched row values of right hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 LEFT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|b|4|null|null|
+
+## 3. RIGHT JOIN:
+RIGHT JOIN returns all the values from the right hand table of the condition and returns only the matching values from the left hand table. The unmatched row values of left hand table will be represented as NULL.
+```sql
+SELECT * FROM TABLE1 RIGHT JOIN TABLE2 ON TABLE1.A=TABLE2.A;
+```
+#### Result:
+
+|A|Z|A|X|
+|---|---|---|---|
+|a|1|a|10|
+|d|5|d|30|
+|null|null|e|17|
+
+## 4. CROSS JOIN:
+CROSS JOIN matches each row of the first table with every row of the second table.
+```sql
+SELECT A,Z,X from TABLE1 CROSS JOIN TABLE2;
+```
+#### Result:
+
+|A|Z|X|
+|---|---|---|
+|a|1|10|
+|a|1|17|
+|a|1|30|
+|b|4|10|
+|b|4|17|
+|b|4|30|
+|d|5|10|
+|d|5|17|
+|d|5|30|
diff --git a/postgresql/triggers/triggers.md b/postgresql/triggers/triggers.md
new file mode 100644
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--- /dev/null
+++ b/postgresql/triggers/triggers.md
@@ -0,0 +1,44 @@
+
+Trigger is a set of actions which gets executed automatically when a specific event occurs like INSERT, UPDATE or DELETE.
+
+## How to create a TRIGGER?
+
+#### Syntax:
+```sql
+CREATE TRIGGER trigger_name [BEFORE/AFTER] [INSERT/UPDATE/DELETE]
+    ON tbl_name 
+    [
+     --trigger_body
+    ];
+```
+
+#### Example:
+In the below example, we will see ORDER_HISTORY table automatically gets updated after a new record is inserted into ORDERS table. This is an example of AFTER INSERT, similarly we can use triggers for INSERT/UPDATE/DELETE events and based on the requirement you can choose whether the update on the second table should happen BEFORE or AFTER.
+
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+```
+```sql
+CREATE TRIGGER ORD_HIS AFTER INSERT ON ORDERS  INSERT INTO ORDER_HISTORY values( new.ORDERID,new.ITEM,new.BILL_AMOUNT,NOW());
+```
+Now insert values into ORDERS table:
+```sql
+INSERT INTO ORDERS VALUES(123,'C10','MANGO',5);
+```
+Check the ORDER_HISTORY table which will yield below results:
+
+|ORDERID|ITEM|WALLETAMOUNT|DATE_OF_ORDER|
+|---|---|---|---|
+|123|Mango|5.00|2019-10-01|
+
+## How to delete a TRIGGER?
+
+#### Syntax: 
+```sql
+DROP TRIGGER trigger_name;
+```
+In the above example, if we want to delete the trigger we created,
+```sql 
+DROP TRIGGER  ORD_HIS;
+```
diff --git a/postgresql/views/views.md b/postgresql/views/views.md
new file mode 100644
index 0000000..4d916e7
--- /dev/null
+++ b/postgresql/views/views.md
@@ -0,0 +1,36 @@
+
+View is a query which is stored in the database to reuse later. Views are nothing but a pseudo table. Views can contain a complex query which retrieves data from multiple tables. So, instead of writing a complex query everytime, you can simply create a view and call it when ever required.
+
+#### Syntax:
+```sql
+Creating a View:
+CREATE VIEW View_name AS 
+Query;
+```
+
+#### How to call view:
+```sql
+SELECT * FROM View_name;
+```
+#### Altering a View:
+```sql
+ALTER View View_name AS 
+Query;
+```
+#### Deleting a View:
+```sql
+DROP VIEW View_name;
+```
+#### Example:
+Let us consider the below two tables:
+```sql
+CREATE TABLE ORDERS(ORDERID INT,CUSTOMERID VARCHAR(50),ITEM VARCHAR(50),BILL_AMOUNT DECIMAL(10,2));
+CREATE TABLE ORDER_HISTORY(ORDERID VARCHAR(50),ITEMS VARCHAR(50),BILL_AMOUNT DECIMAL(10,2),DATE_OF_ORDER DATE);
+
+-- creating view
+CREATE VIEW ORD_VIEW AS SELECT O.ORDERID,O.CUSTOMERID,H.ITEM,H.DATE_OF_ORDER from ORDERS O INNER JOIN ORDER_HISTORY H; 
+
+-- to display the view
+select * from ORD_VIEW;
+```
+In this way, we can also restrict the visibility of columns of the table to the users.
\ No newline at end of file
diff --git a/redis/advanced-topics/transactions.md b/redis/advanced-topics/transactions.md
new file mode 100644
index 0000000..42290d6
--- /dev/null
+++ b/redis/advanced-topics/transactions.md
@@ -0,0 +1,63 @@
+### Transactions
+
+Redis transactions allow execution of multiple commands in a single step.
+*  All the commands are serialized and executed in a sequential manner.
+*  Redis transactions are atomic, either all the commands or none are processed.
+*  Transaction is a single isolate operation, No other request is served in the middle of transaction.
+*  MULTI, EXEC, DISCARD AND WATCH are commands used in a transaction.
+
+## MULTI
+*  `MULTI` command is used to initiate a redis transaction .
+*  At this point user can issue mulitple commands.  
+*  Instead of executing all the commands, redis will keep them in queue.
+
+
+## EXEC
+* Executes all previously queued commands in a transaction and restores the state to normal.
+
+```sh
+> MULTI
+OK
+> INCR alpha
+QUEUED
+> INCR beta
+QUEUED
+> EXEC
+1) (integer) 1
+2) (integer) 1
+```
+
+* Returns an array of replies, in which every element is reply of each command in the same order.
+
+## DISCARD
+* `DISCARD` is used to abort the transaction.
+* State of the connection is restored to normal.
+
+```sh
+> MULTI
+OK
+> INCR alpha
+QUEUED
+> DISCARD
+OK
+> GET alpha
+"1"
+```
+
+## WATCH
+* `WATCH` Marks the given key to be watched for conditional exection of transaction.
+* Transaction will be only be performed if none of the watched keys are modified.
+
+```sh
+WATCH mykey
+val = GET mykey
+val = val + 1
+MULTI
+SET mykey $val
+EXEC
+```
+
+* In the above code, if another client modifies the result of val in between WATCH and EXEC, transaction will fail.
+
+## UNWATCH
+* `UNWATCH` flushes all the previously watched keys in transaction.
\ No newline at end of file
diff --git a/redis/index.json b/redis/index.json
new file mode 100644
index 0000000..f78885d
--- /dev/null
+++ b/redis/index.json
@@ -0,0 +1,60 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title": "What is Redis",
+                "path": "what-is-redis"
+            },
+            {
+                "title": "Key Concepts",
+                "path": "key-concepts"
+            },
+            {
+                "title": "Installation",
+                "path": "installation"
+            }
+        ]
+    },
+    {
+        "title": "Redis Operations",
+        "path": "redis-operations",
+        "links": [
+            {
+                "title": "Redis Commands",
+                "path": "commands"
+            },
+            {
+                "title": "String",
+                "path": "string-commands"
+            },
+            {
+                "title": "List",
+                "path": "list-commands"
+            },
+            {
+                "title": "Set",
+                "path": "set-commands"
+            },
+            {
+                "title": "Hash",
+                "path": "hash-commands"
+            },
+            {
+                "title": "Sorted-Set",
+                "path": "sorted-set-commands"
+            }
+        ]
+    },
+    {
+        "title": "Advanced Topics",
+        "path": "advanced-topics",
+        "links": [
+            {
+                "title": "Transactions",
+                "path": "transactions"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/redis/introduction/installation.md b/redis/introduction/installation.md
new file mode 100644
index 0000000..e34f0bf
--- /dev/null
+++ b/redis/introduction/installation.md
@@ -0,0 +1,38 @@
+
+##  Installation using Homebrew
+ ```sh
+ brew install redis
+ ```
+## Installing using Docker
+```sh
+docker pull redis
+```
+
+## Installing on Windows
+
+Download application from https://redis.io/download 
+
+## Installing on Ubuntu
+```sh
+sudo apt-get install redis-server
+```
+
+## Try Redis Online
+
+https://try.redis.io
+
+
+## Start Redis 
+```sh
+redis-server
+```
+
+## Connect to Redis
+```sh
+redis-cli
+```
+```sh
+redis 127.0.0.1:6379> ping 
+PONG
+```
+If you receive PONG, then redis is succesfully installed on your system.
diff --git a/redis/introduction/key-concepts.md b/redis/introduction/key-concepts.md
new file mode 100644
index 0000000..59fdeae
--- /dev/null
+++ b/redis/introduction/key-concepts.md
@@ -0,0 +1,22 @@
+## Key Concepts
+
+
+### Redis Keys
+
+Redis keys are binary-safe which means you can use key from a string like "tutorial" to content of JPEG file.
+
+Maximum allowed key size is 512MB.
+
+Redis has a feature to keep expiration time of a key, which is limited time to live after which key is automatically destroyed. It is optional.
+
+### Redis Data Types
+
+There are 5 different data types supported by redis.
+
+1. String
+1. list
+1. set
+1. zset or sorted sets
+1. hash
+
+
diff --git a/redis/introduction/what-is-redis.md b/redis/introduction/what-is-redis.md
new file mode 100644
index 0000000..87fe3aa
--- /dev/null
+++ b/redis/introduction/what-is-redis.md
@@ -0,0 +1,5 @@
+Redis is a key-valued Pair NoSQL database. It was written in C. Redis is an opensource in-memory data structure store.
+
+Redis is mainly used as Message Broker or cache.
+
+
diff --git a/redis/redis-operations/commands.md b/redis/redis-operations/commands.md
new file mode 100644
index 0000000..1356386
--- /dev/null
+++ b/redis/redis-operations/commands.md
@@ -0,0 +1,66 @@
+### Insert Key - SET
+
+`SET` command is used to create key-value data .
+```sh
+> SET KEY VALUE
+```
+Key will be created if it does not exist, or else value will be updated.
+
+
+```sh
+> SET whichdb redis
+OK
+```
+
+### Reading Key - GET, MGET
+
+`GET` command is used to read value in key.
+
+```sh
+> GET whichdb
+"redis"
+```
+
+`MGET` command is used to read values of multiple keys.
+```sh
+> MGET key1 key2
+```
+
+### Rename keys - RENAME
+
+```sh
+RENAME whichdb finddb
+```
+### Deleting Key
+
+`DEL` command is used to delete single/multiple keys. Key is ignored if it does not exist. 
+
+```sh
+> DEL whichdb
+(integer) 1
+> DEL key1 key2 key3
+```
+### Type of Key 
+
+```sh
+> type whichdb
+"string"
+```
+
+
+### EXPIRE Keys - EXPIRE
+`EXPIRE` command is used to set timeout for a key in seconds.
+
+```sh
+> EXPIRE key 10
+(integer) 1
+```
+
+`TTL` command is used to find remaining time to live for the key.
+
+```sh
+> TTL key
+(integer) 10
+```
+
+Reference to all the commands in redis: https://redis.io/commands
\ No newline at end of file
diff --git a/redis/redis-operations/hash-commands.md b/redis/redis-operations/hash-commands.md
new file mode 100644
index 0000000..54cabf4
--- /dev/null
+++ b/redis/redis-operations/hash-commands.md
@@ -0,0 +1,63 @@
+## Hashes
+
+Hashes is a collection of maps between string fields and string values.
+* Hashes are mainly used to represent objects.
+* Addition and Retreiving the elements are performed in constant time.
+
+### Insertion  - HMSET, HSET
+* `HMSET` key fields1 value1 field2 value2 - Used to set multiple hash fields to multiple values.
+* `HGETALL` key - Returns all the fields and values from the given key.
+* `HSET` key field value - Used to set given hash field to value.
+* `HGET` key field- Returns the value of given hash field in key.
+
+
+```sh
+> HMSET redis name "redis" type "Message Broker" website "https://redis.io/" 
+(integer) 3
+> HGETALL redis
+1) "name"
+2) "redis"
+3) "type"
+4) "Message Broker"
+5) "website"
+6) "https://redis.io/"
+
+> HSET redis opensource "true"
+(integer) 1
+> HGET redis opensource
+"true"
+```
+### Deletion - HDEL
+* `HDEL` key field1 field2 - Removes the specified fields from given key.
+
+```sh
+> HDEL redis opensource
+(integer) 1
+> HGETALL redis
+1) "name"
+2) "redis"
+3) "type"
+4) "Message Broker"
+5) "website"
+6) "https://redis.io/"
+```
+
+### Miscellaneous 
+* `HKEYS` key - Returns all the fields stored in a key.
+* `HVALS` key -  Retunr all the values in key.
+* `HLEN` key - Returns the number of fields in key.
+
+```sh
+> HKEYS redis
+1) "name"
+2) "type"
+3) "website"
+
+> HVALS redis
+1) "redis"
+2) "Message Broker"
+3) "https://redis.io/"
+
+> HLEN redis
+3
+```
diff --git a/redis/redis-operations/list-commands.md b/redis/redis-operations/list-commands.md
new file mode 100644
index 0000000..9bd5020
--- /dev/null
+++ b/redis/redis-operations/list-commands.md
@@ -0,0 +1,102 @@
+## List
+
+List is a collection of string elements stored in order of insertion. 
+* Redis Lists are implemented via Linkedin Lists.
+* Insertion of element is performed in constant time regardless of size of list.
+* Allows Duplicate values.
+
+### Insertion / Pushing - LPUSH, RPUSH
+* `LPUSH` key value - Inserts the value to the key at the head.
+* `RPUSH` key value - Inserts the value to key at the tail.
+* `LRANGE` key start stop - Displays all the elements in the key with given range.
+
+
+```sh
+> LPUSH whichdb redis
+(integer) 1
+> LRANGE whichdb 0 -1
+1) "redis"
+
+> LPUSH whichdb rabbitmq oracle
+(integer) 3
+> LRANGE whichdb 0 -1
+1) "oracle"
+2) "rabbitmq"
+3) "redis"
+```
+
+```sh
+> RPUSH whichdb mysql
+(integer) 4
+> LRANGE whichdb 0 -1
+1) "oracle"
+2) "rabbitmq"
+3) "redis"
+4) "mysql"
+
+> RPUSH whichdb mongo elastic
+(integer) 6
+> LRANGE whichdb 0 -1
+1) "oracle"
+2) "rabbitmq"
+3) "redis"
+4) "mysql"
+5) "mongo"
+6) "elastic"
+```
+
+### Deletion / Popping - LPOP, RPOP
+* `LPOP` key - retrieves and deletes the first element at the head.
+* `RPOP` key - retrieve and deletes the first element at the tail.
+
+```sh
+> LPOP whichdb 
+"oracle"
+> LRANGE whichdb 0 -1
+1) "rabbitmq"
+2) "redis"
+3) "mysql"
+4) "mongo"
+5) "elastic"
+
+> rpop whichdb
+"elastic"
+> LRANGE whichdb 0 -1
+1) "rabbitmq"
+2) "redis"
+3) "mysql"
+4) "mongo"
+```
+
+### Miscellaneous 
+* `LSET` key index value - Sets the list element of index to value.
+* `LTRIM` key start stop -  Trims the list so that only given range of elements exist.
+* `LINSERT` key BEFORE|AFTER index value - Inserts the value in the list at either after or before the index.
+* `LLEN` key - Displays the length of list.
+
+```sh
+> LSET whichdb 3 postgresql
+OK
+> LRANGE whichdb 0 -1
+1) "rabbitmq"
+2) "redis"
+3) "mysql"
+4) "postgresql"
+5) "elastic"
+
+> LTRIM whichdb 0 1
+OK
+> LRANGE whichdb 0 -1
+1) "rabbitmq"
+2) "redis"
+
+> LINSERT whichdb AFTER redis mysql
+3
+> LRANGE whichdb 0 -1
+1) "rabbitmq"
+2) "redis"
+3) "mysql"
+
+> LLEN whichdb
+(integer) 3
+```
diff --git a/redis/redis-operations/set-commands.md b/redis/redis-operations/set-commands.md
new file mode 100644
index 0000000..df6c317
--- /dev/null
+++ b/redis/redis-operations/set-commands.md
@@ -0,0 +1,93 @@
+## Sets
+
+Set is a collection of unordered, non-repeating string values.
+* Redis Sets are implemented using Hash tables.
+* Adding, Insertion and checking existence of value is performed in constant time irrespective of size.
+* Redis sets support union, intersection and differences of sets.
+* Can store a maximum of 4 billion elements for a set.
+
+### Insertion  - SADD
+* `SADD` key value1 value2 - Inserts the values to the key.
+* `SMEMBERS` key - Return all the elements in the set.
+
+```sh
+> SADD odd 5 3 3 1
+(integer) 3
+> SMEMBERS odd
+1) "1"
+2) "3"
+3) "5"
+
+> SADD prime 2 3 5 5 7
+(integer) 4
+> SMEMBERS prime
+1) "2"
+2) "3"
+3) "5"
+4) "7"
+```
+### Deletion - SREM, SPOP
+* `SREM` key1 value1 value2 - Removes the specified members from the set for given key.
+* `SPOP` key - Removes and returns one or more members from the key.
+
+```sh
+> SREM prime 5
+1
+> SMEMBERS prime
+1) "2"
+2) "3"
+3) "7"
+
+> SPOP prime
+"3"
+> SMEMBERS prime
+1) "2"
+2) "7"
+```
+
+### Set Operations - SINTER, SUNION, SDIFF
+* `SINTER` key1 key2 - Returns the members of the set resulting from intersection of all given sets.
+* `SUNION` key1 key2 - Returns the members of the set resulting from union of all given sets.
+* `SDIFF` key1 key2 - Returns the members of the set resulting from difference of all given sets.
+
+```sh
+> SINTER odd prime 
+1) "3"
+2) "5"
+
+> SUNION odd prime
+1) "2"
+2) "3"
+3) "5"
+4) "7"
+
+> SDIFF odd prime
+1) "2"
+2) "7"
+```
+
+### Miscellaneous 
+* `SCARD` key - Returns the number of elements stored in a key.
+* `SRANDMEMBER` key count-  Retunr the given number(count) of random elements in key.
+* `SISMEMBER` key value- Returns if a given element already exist in the provided set or not.
+
+```sh
+> SMEMBERS odd
+1) "1"
+2) "3"
+3) "5"
+4) "7"
+
+> SCARD odd
+4
+
+> SRANDMEMBER odd 2
+1) "7"
+2) "2"
+
+> SISMEMBER odd 3
+(integer) 1
+
+> SISMEMBER odd 4
+(integer) 0
+```
diff --git a/redis/redis-operations/sorted-set-commands.md b/redis/redis-operations/sorted-set-commands.md
new file mode 100644
index 0000000..7d79919
--- /dev/null
+++ b/redis/redis-operations/sorted-set-commands.md
@@ -0,0 +1,138 @@
+## Sorted Sets
+
+Sorted set are similar to set, a collection of unique strings, the difference is every member in a sorted set is associated with a score which is used to make sorted set ordered from smallest to greatest score.
+* Redis Sorted Sets are implemented using Hash tables.
+* Adding, Insertion or Updation of elements can be performed in a fast way O(logn)
+* Sorted sets are used to index data stored in redis.
+* It is the most advanced data type in redis.
+
+### Insertion  - ZADD
+* `ZADD` key [NX|XX] [CH] [INCR] score value - Inserts the value to the key with the given score based upon given option.
+    * NX - Only Add elements if they do not exist, Ignore if they already exist.
+    * XX - Only update elements if they already exist, Ignore if they  do not exist.
+    * CH - Modifies the return value to number of elements changed ie.updated or added.
+    * INCR - Increments the score of given element. This only works for score-element pair.
+* `ZRANGE` key start stop [WITHSCORES] - Return all the elements in the sorted set with in the specified range.
+
+```sh
+> ZADD whichdb 1 "redis" 2 "rabbitmq" 3 "oracle"
+(integer) 1
+> ZRANGE whichdb 0 -1 WITHSCORES
+1) "redis"
+2) 1.0
+3) "rabbitmq"
+4) 2.0
+5) "oracle"
+6) 3.0
+
+> ZADD NX whichdb 1 "redis" 
+(integer) 0
+> ZADD NX whichdb 4 "mysql" 
+(integer) 1
+> ZRANGE whichdb 0 -1 WITHSCORES
+1) "redis"
+2) 1.0
+3) "rabbitmq"
+4) 2.0
+5) "oracle"
+6) 3.0
+7) "mysql"
+8) 4.0
+```
+```sh
+> ZADD whichdb XX 5 "redis"
+(integer) 0
+> ZRANGE whichdb 0 -1 WITHSCORES
+1) "rabbitmq"
+2) 2.0
+3) "oracle"
+4) 3.0
+5) "mysql"
+6) 4.0
+7) "redis"
+8) 5.0
+
+> ZADD whichdb CH 1 "redis"  5 "mysql"
+(integer) 2
+> ZRANGE whichdb 0 -1 WITHSCORES
+1) "redis"
+2) 1.0
+3) "rabbitmq"
+4) 2.0
+5) "oracle"
+6) 3.0
+7) "mysql"
+8) 5.0
+
+> ZADD whichdb INCR 1 "redis"
+(integer) 2
+> ZRANGE whichdb 0 -1 WITHSCORES
+1) "rabbitmq"
+2) 2.0
+3) "redis"
+4) 2.0
+5) "oracle"
+6) 3.0
+7) "mysql"
+8) 5.0
+```
+
+### Deletion - ZREM, ZREMRANGEBYRANK, ZREMRANGEBYSCORE
+* `ZREM` key member - Removes the specified members from the sorted set for given key.
+* `ZREMRANGEBYRANK` key start stop - Removes all the elements with in specified rank from the key.
+* `ZREMRANGEBYSCORE` key start stop - Removes all the elements with in specified score from the key.
+
+```sh
+> ZREM whichdb "mysql"
+1
+> ZRANGE whichdb 0 -1 WITHSCORES
+1) "rabbitmq"
+2) 2.0
+3) "redis"
+4) 2.0
+5) "oracle"
+6) 3.0
+
+> ZREMRANGEBYRANK whichdb 1 2
+(integer) 2
+> ZRANGE whichdb 0 -1 WITHSCORES
+1) "rabbitmq"
+2) 2.0
+
+> ZREMRANGEBYRANK whichdb 1 2
+(integer) 1
+> ZRANGE whichdb 0 -1 WITHSCORES
+(empty list or set)
+```
+
+### Miscellaneous 
+* `ZCARD` key - Returns the number of elements stored in a sorted set.
+* `ZCOUNT` key min max-  Returns the number(count) of elements in sorted set with scores in given range.
+* `ZRANK` key member- Returns the rank or index of a member in a sorted set.
+* `ZSCORE` key member- Returns the score of a member in a sorted set.
+
+```sh
+> ZRANGE sports 0 -1 withscores
+1) "cricket"
+2) 1.0
+3) "baseball"
+4) 3.0
+5) "football"
+6) 6.0
+7) "hockey"
+8) 6.0
+9) "basketball"
+10) 9.0
+
+> ZCARD sports
+5
+
+> ZCOUNT sports 3 7
+3
+
+> ZRANK sports "hockey"
+3
+
+> ZSCORE sports "basketball"
+9.0
+```
diff --git a/redis/redis-operations/string-commands.md b/redis/redis-operations/string-commands.md
new file mode 100644
index 0000000..8b5dd5c
--- /dev/null
+++ b/redis/redis-operations/string-commands.md
@@ -0,0 +1,84 @@
+## String
+
+It is basic data type which can store Strings, Integer and floating points
+
+```sh
+> SET rank 1
+OK
+> SET average 4.5
+OK
+> SET name redis
+OK
+```
+
+Redis provides basic commands to manage strings
+
+### Increment - INCR, INCRBY, INCRBYFLOAT
+
+INCR, INCRBY and INCRBYFLOAT are used to increase the value of integer or float value stored in a string
+
+* `INCR` key- Increments the value of integer by 1
+* `INCRBY` key- Increments the value of integer by given amount
+* `INCRBYFLOAT` key value- Increments the floating value by given amount ( Applicable for integers also)
+```sh
+> INCR rank
+(integer) 2
+> GET rank
+"2"
+
+> INCRBY rank 3
+(integer) 5
+> GET rank
+"5"
+
+> INCRBYFLOAT average 2.3
+6.8
+> GET average
+"6.8"
+```
+
+### Decrement - DECR, DECRBY
+
+DECR, DECRBY are used to decrease the value of integer  value stored in a string
+
+* `DECR` key- Decrements the value of integer by 1
+* `DECRBY` key value- Decrements the value of integer by given amount
+
+```sh
+> GET rank
+"5"
+> DECR rank
+(integer) 4
+> GET rank
+"4"
+
+> DECRBY rank 3
+(integer) 1
+> GET rank
+"1"
+```
+
+### String Manipulation - SETRANGE, APPEND, STRLEN
+
+SETRANGE, APPEND, SETRANGE are used manipulation of value stored in a string
+
+* `SETRANGE` key offest value- Overwrites the part of string at the given offset with given value
+* `APPEND` key value- Appends the value to the string
+* `STRLEN` key- Determines the length of string
+
+```sh
+> GET name
+"redis"
+> SETRANGE name 1 ichard
+7
+> GET name
+"richard"
+
+> APPEND name son
+10
+> GET name
+"richardson"
+
+> STRLEN name
+(integer) 10
+```
\ No newline at end of file
diff --git a/sql-lite/crud-operations/attach-database.md b/sql-lite/crud-operations/attach-database.md
new file mode 100644
index 0000000..c89369c
--- /dev/null
+++ b/sql-lite/crud-operations/attach-database.md
@@ -0,0 +1,12 @@
+This query is used to select a single database from available set. all the following SQLite3 statements will be executed under the attached database.
+
+```sh
+ATTACH DATABASE 'DatabaseName' as 'Alias'
+```
+
+This command is used to list the databases created.
+```sh
+.databases
+```
+
+Database files are created in root folder.
\ No newline at end of file
diff --git a/sql-lite/crud-operations/create-data.md b/sql-lite/crud-operations/create-data.md
new file mode 100644
index 0000000..85066fb
--- /dev/null
+++ b/sql-lite/crud-operations/create-data.md
@@ -0,0 +1,23 @@
+`INSERT INTO` query is used to add new rows into table.
+
+### Syntax 1
+
+If you are not inserting all the columns in a row, you need to mention the column name also.
+
+```sh
+INSERT INTO TABLE_NAME [(column1, column2, column3,...columnN)]    
+VALUES (value1, value2, value3,...valueN);   
+```
+
+### Syntax 2
+
+If you are inserting all the columns in a row, you need need mention the column names.
+
+```sh
+INSERT INTO TABLE_NAME VALUES (value1,value2,value3,...valueN);   
+```
+
+Eg:
+```sh
+INSERT INTO STUDENT VALUES (1, 'John', 18, 85);   
+```
\ No newline at end of file
diff --git a/sql-lite/crud-operations/create-database.md b/sql-lite/crud-operations/create-database.md
new file mode 100644
index 0000000..73ae683
--- /dev/null
+++ b/sql-lite/crud-operations/create-database.md
@@ -0,0 +1,16 @@
+`sqlite3` query is used to create a database.
+
+```sh
+sqlite3 <db_name.db>
+```
+#### Example
+```sh
+sqlite3 students.db
+```
+
+This command is used to list the databases created.
+```sh
+.databases
+```
+
+Database files are created in root folder.
\ No newline at end of file
diff --git a/sql-lite/crud-operations/create-table.md b/sql-lite/crud-operations/create-table.md
new file mode 100644
index 0000000..1fd7e4c
--- /dev/null
+++ b/sql-lite/crud-operations/create-table.md
@@ -0,0 +1,29 @@
+`CREATE TABLE` query is used to create a table in the database.
+
+```sh
+CREATE TABLE table_name(  
+   column1 datatype,  
+   column2 datatype,  
+   column3 datatype,  
+   .....  
+   columnN datatype,  
+);  
+```
+#### Example
+
+```sh
+CREATE TABLE STUDENT(  
+   ID INT PRIMARY KEY     NOT NULL,  
+   NAME           TEXT    NOT NULL,  
+   AGE            INT     NOT NULL,  
+   TOTAL          INT 
+);   
+
+```
+
+This command is used to list the databases created.
+```sh
+.databases
+```
+
+Database files are created in root folder.
\ No newline at end of file
diff --git a/sql-lite/crud-operations/deleting-data.md b/sql-lite/crud-operations/deleting-data.md
new file mode 100644
index 0000000..bf5106a
--- /dev/null
+++ b/sql-lite/crud-operations/deleting-data.md
@@ -0,0 +1,12 @@
+`DELETE` query is used to delete the existing records in a table. It is used with WHERE clause otherwise all the data will be deleted.
+
+## Deleting based on condition
+```sh
+DELETE FROM table_name  
+WHERE [conditions]
+```
+
+## Deleting all the records from a table.
+```sh
+DELETE FROM table_name;   
+```
\ No newline at end of file
diff --git a/sql-lite/crud-operations/detach-database.md b/sql-lite/crud-operations/detach-database.md
new file mode 100644
index 0000000..466c0f2
--- /dev/null
+++ b/sql-lite/crud-operations/detach-database.md
@@ -0,0 +1,7 @@
+`DETACH` query is used to detach alias-based database which was previously attached through ATTACH statement 
+
+```sh
+DETACH DATABASE 'Alias'
+```
+
+Even if a single database is attached with multiple aliases, DETACH will disconnect only with given name.
\ No newline at end of file
diff --git a/sql-lite/crud-operations/reading-data.md b/sql-lite/crud-operations/reading-data.md
new file mode 100644
index 0000000..81bd486
--- /dev/null
+++ b/sql-lite/crud-operations/reading-data.md
@@ -0,0 +1,28 @@
+## Reading all documents from a collection
+`SELECT` is used to read fetch data from a table in database.
+
+## SELECT All the data in table
+
+```sh
+SELECT * FROM table_name;   
+```
+
+## SELECT Required columns in a table
+
+```sh
+SELECT column1, column2, columnN FROM table_name;   
+```
+
+## SELECT based on condition
+
+```sh
+SELECT column1, column2, columnN   
+FROM table_name  
+WHERE [condition]  
+```
+
+#### Example
+
+```sh
+SELECT * FROM STUDENT WHERE ID >= 10 AND TOTAL >= 90;   
+```
\ No newline at end of file
diff --git a/sql-lite/crud-operations/updating-data.md b/sql-lite/crud-operations/updating-data.md
new file mode 100644
index 0000000..f879710
--- /dev/null
+++ b/sql-lite/crud-operations/updating-data.md
@@ -0,0 +1,14 @@
+`UPDATE` query is used to update the existing data in the table. It is used with where clause otherwise all the rows will be updated 
+
+```sh
+UPDATE table_name  
+SET column1 = value1, column2 = value2...., columnN = valueN  
+WHERE [condition];   
+```
+
+#### Example
+
+```sh
+UPDATE STUDENT SET AGE = AGE + 2 WHERE ID = 5;   
+
+```
\ No newline at end of file
diff --git a/sql-lite/index.json b/sql-lite/index.json
new file mode 100644
index 0000000..a4061fe
--- /dev/null
+++ b/sql-lite/index.json
@@ -0,0 +1,58 @@
+[
+    {
+        "title": "Introduction",
+        "path": "introduction",
+        "links": [
+            {
+                "title": "What is SQLite",
+                "path": "what-is-sqlite"
+            },
+            {
+                "title": "Key Concepts",
+                "path": "key-concepts"
+            },
+            {
+                "title": "Installation",
+                "path": "installation"
+            }
+        ]
+    },
+    {
+        "title": "CRUD Operations",
+        "path": "crud-operations",
+        "links": [
+            {
+                "title": "Creating Database",
+                "path": "create-database"
+            },
+            {
+                "title": "Attach Database",
+                "path": "attach-database"
+            },
+            {
+                "title": "Detach Database",
+                "path": "detach-database"
+            },
+            {
+                "title": "Creating Table",
+                "path": "create-table"
+            },
+            {
+                "title": "Inserting Data",
+                "path": "create-data"
+            },
+            {
+                "title": "Reading Data",
+                "path": "reading-data"
+            },
+            {
+                "title": "Updating Data",
+                "path": "updating-data"
+            },
+            {
+                "title": "Deleting Data",
+                "path": "deleting-data"
+            }
+        ]
+    }
+]
\ No newline at end of file
diff --git a/sql-lite/introduction/installation.md b/sql-lite/introduction/installation.md
new file mode 100644
index 0000000..743bd34
--- /dev/null
+++ b/sql-lite/introduction/installation.md
@@ -0,0 +1,6 @@
+SQLite is zero-configuration/serverless which requires no complex setup. You can install on your system in the following way. 
+
+## Installing on MacOS, Windows, Android
+
+Go to SQLite website https://www.sqlite.org/download.html. Download required binaries dependending on the os.
+Extract the download file and open the directory and open sqlite3.
diff --git a/sql-lite/introduction/key-concepts.md b/sql-lite/introduction/key-concepts.md
new file mode 100644
index 0000000..4ba829c
--- /dev/null
+++ b/sql-lite/introduction/key-concepts.md
@@ -0,0 +1,8 @@
+## Zero configuration
+SQLite works without any installation, setup. There is no server which needs to started, stopped or configured. It required no configuration files. No actions are required to recover after a system crash.
+
+## Serverless
+Most of other database are implemented as a separate server. Users who wants to communicate with the server needs to send requests to the server and receive the results. With sqlite, the process that wants to access the database read and writes directly from the database file on to disk.
+
+## Single Database File
+SQLite database is a single ordinary disk file that can be located in the directory hierarchy. Database files can be easily transferred to USB memory stick or email.
diff --git a/sql-lite/introduction/what-is-sqlite.md b/sql-lite/introduction/what-is-sqlite.md
new file mode 100644
index 0000000..64eab0f
--- /dev/null
+++ b/sql-lite/introduction/what-is-sqlite.md
@@ -0,0 +1,2 @@
+SQLite is a open source library that implements self-contained, serverless, zero configuration and transactional SQL database engine.
+It is the mostly widely deployed database in the world.