docs(notes): add MySQL to IBM DB2 migration guide

Author: lzwjava

notes/2025-03-24-mysql-ibmdb2-en.md

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+---
+title: MySQL and IBM Db2
+lang: en
+layout: post
+audio: false
+translated: false
+generated: true
+---
+
+MySQL and IBM Db2 are both robust relational database management systems (RDBMS), but they differ in architecture, syntax, features, and use cases. Below is a comparison focusing on aspects like table schema, table naming, `GROUP BY`, and other operations:
+
+---
+
+### 1. **Table Schema**
+- **MySQL**:
+  - Uses a schema structure tied to databases. Each database contains tables, and schemas are essentially synonymous with databases.
+  - Schema creation is straightforward with `CREATE DATABASE` or `CREATE SCHEMA` (the latter is an alias for the former).
+  - Example: `CREATE DATABASE mydb; USE mydb; CREATE TABLE mytable (id INT, name VARCHAR(50));`
+  - MySQL supports storage engines (e.g., InnoDB, MyISAM), which can affect table behavior like transaction support or indexing.
+
+- **IBM Db2**:
+  - Schemas are logical groupings within a database. A single Db2 database can contain multiple schemas, and tables belong to a specific schema.
+  - Schema creation: `CREATE SCHEMA myschema;` followed by table creation within it, e.g., `CREATE TABLE myschema.mytable (id INT, name VARCHAR(50));`.
+  - Db2 enforces stricter schema management, often used in enterprise environments for logical separation of objects.
+
+**Key Difference**: MySQL ties schema to the database level, while Db2 allows multiple schemas within a database, offering finer-grained organization.
+
+---
+
+### 2. **Table Naming**
+- **MySQL**:
+  - Table names are case-sensitive on Unix-like systems but case-insensitive on Windows, depending on the file system and configuration (`lower_case_table_names` setting).
+  - Maximum table name length is 64 characters.
+  - Qualified as `database_name.table_name` (e.g., `mydb.mytable`).
+
+- **IBM Db2**:
+  - Table names are generally case-insensitive unless explicitly quoted (e.g., `"MyTable"`), and they are stored in uppercase by default.
+  - Maximum table name length is 128 characters in newer versions (e.g., Db2 11.5).
+  - Qualified as `schema_name.table_name` (e.g., `myschema.mytable`).
+
+**Key Difference**: MySQL’s table naming is tied to the database and has shorter length limits, while Db2 uses schema-based qualification and supports longer names.
+
+---
+
+### 3. **GROUP BY**
+- **MySQL**:
+  - Historically lenient with `GROUP BY`. Before version 5.7, MySQL allowed selecting non-aggregated columns not listed in `GROUP BY`, returning arbitrary values for those columns (though this behavior can be controlled with `ONLY_FULL_GROUP_BY` mode).
+  - Example: `SELECT id, name, COUNT(*) FROM mytable GROUP BY id;` works fine even if `name` isn’t aggregated or grouped (in older versions or with relaxed settings).
+  - Supports `GROUP BY` with `ROLLUP` for summary rows.
+
+- **IBM Db2**:
+  - Stricter adherence to SQL standards. All non-aggregated columns in the `SELECT` list must appear in the `GROUP BY` clause.
+  - Example: `SELECT id, name, COUNT(*) FROM mytable GROUP BY id, name;`—omitting `name` from `GROUP BY` would raise an error.
+  - Offers advanced grouping features like `GROUPING SETS`, `CUBE`, and `ROLLUP` for complex aggregations.
+
+**Key Difference**: MySQL is more permissive with `GROUP BY` (depending on configuration), while Db2 enforces stricter SQL standard compliance.
+
+---
+
+### 4. **Other Operations**
+- **Data Types**:
+  - **MySQL**: Offers types like `TINYINT`, `MEDIUMTEXT`, and `JSON`. No native boolean type (uses `TINYINT(1)`).
+  - **Db2**: Includes types like `DECFLOAT`, `XML`, and `BOOLEAN`. More enterprise-focused options.
+
+- **Indexes**:
+  - **MySQL**: Supports B-tree, hash (in some engines), and full-text indexes. InnoDB uses clustered indexes by default for primary keys.
+  - **Db2**: Supports B-tree, unique, and bitmap indexes, with advanced options like index compression and partitioning for large-scale data.
+
+- **Transactions**:
+  - **MySQL**: Transaction support depends on the storage engine (InnoDB supports ACID, MyISAM does not).
+  - **Db2**: Full ACID compliance across all tables, designed for high-reliability enterprise use.
+
+- **Joins**:
+  - Both support standard SQL joins (`INNER`, `LEFT`, `RIGHT`, `FULL OUTER`), but Db2 has better optimization for complex joins in large datasets, while MySQL is simpler and lighter for smaller-scale applications.
+
+- **Stored Procedures and Functions**:
+  - **MySQL**: Uses a simpler procedural language with basic control structures.
+  - **Db2**: Supports a richer SQL PL language, including advanced error handling and integration with external languages (e.g., Java, C).
+
+- **Performance and Scalability**:
+  - **MySQL**: Optimized for web applications, lightweight, and easy to scale horizontally with replication.
+  - **Db2**: Built for enterprise workloads, with superior support for vertical scaling, parallelism, and large-scale data warehousing.
+
+---
+
+### Summary
+- **MySQL** is simpler, more flexible, and widely used in open-source and web development contexts. It’s less strict with syntax and schema design but may lack some enterprise-grade features.
+- **IBM Db2** is more rigid, standards-compliant, and suited for complex, high-performance enterprise applications, with advanced features for data management and analytics.
+
+If you have a specific operation or use case in mind, let me know, and I can dive deeper!