Create Joins.md

Author: udithaWICK

Joins.md

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+### **SQL Joins**  
+SQL **Joins** are used to **combine rows** from two or more tables based on a **related column**. Joins are essential for **retrieving data** spread across multiple tables.
+
+---
+
+## **1. Types of Joins**
+
+### **1.1 INNER JOIN**  
+Returns **only matching rows** from both tables.  
+
+```sql
+SELECT e.id, e.name, d.department_name
+FROM employees e
+INNER JOIN departments d
+ON e.department_id = d.department_id;
+```
+**Explanation:**
+- Matches rows where **employees.department_id = departments.department_id**.  
+- Rows without matches are **excluded**.
+
+---
+
+### **1.2 LEFT JOIN (LEFT OUTER JOIN)**  
+Returns **all rows from the left table** and **matching rows from the right table**. Non-matching rows in the right table are filled with **NULL**.  
+
+```sql
+SELECT e.id, e.name, d.department_name
+FROM employees e
+LEFT JOIN departments d
+ON e.department_id = d.department_id;
+```
+**Use Case:**  
+- Retrieve all employees, even if they **don’t belong to any department**.
+
+---
+
+### **1.3 RIGHT JOIN (RIGHT OUTER JOIN)**  
+Returns **all rows from the right table** and **matching rows from the left table**. Non-matching rows in the left table are filled with **NULL**.  
+
+```sql
+SELECT e.id, e.name, d.department_name
+FROM employees e
+RIGHT JOIN departments d
+ON e.department_id = d.department_id;
+```
+**Use Case:**  
+- Retrieve **all departments**, even if they have **no employees**.
+
+---
+
+### **1.4 FULL OUTER JOIN**  
+Returns **all rows from both tables**. Non-matching rows in either table are filled with **NULL**.  
+
+**Note:** MySQL does not support **FULL OUTER JOIN** directly, so it can be simulated using **UNION**:
+
+```sql
+SELECT e.id, e.name, d.department_name
+FROM employees e
+LEFT JOIN departments d
+ON e.department_id = d.department_id
+
+UNION
+
+SELECT e.id, e.name, d.department_name
+FROM employees e
+RIGHT JOIN departments d
+ON e.department_id = d.department_id;
+```
+**Use Case:**  
+- Retrieve a **complete list of employees and departments**, showing unmatched records from both sides.
+
+---
+
+### **1.5 CROSS JOIN**  
+Produces a **Cartesian Product** of rows, combining **all rows from both tables**.  
+
+```sql
+SELECT e.name, d.department_name
+FROM employees e
+CROSS JOIN departments d;
+```
+**Use Case:**  
+- Generate all possible **combinations** between employees and departments.
+
+---
+
+### **1.6 SELF JOIN**  
+Joins a table with **itself**.  
+
+```sql
+SELECT e1.name AS employee, e2.name AS manager
+FROM employees e1
+INNER JOIN employees e2
+ON e1.manager_id = e2.id;
+```
+**Use Case:**  
+- Retrieve a **hierarchy** where employees are related to managers in the same table.
+
+---
+
+## **2. Practical Examples**
+
+### **2.1 Find Employees and Their Department Names**
+```sql
+SELECT e.name, e.salary, d.department_name
+FROM employees e
+INNER JOIN departments d
+ON e.department_id = d.department_id;
+```
+
+---
+
+### **2.2 Find Employees Without Departments**
+```sql
+SELECT e.name, e.salary
+FROM employees e
+LEFT JOIN departments d
+ON e.department_id = d.department_id
+WHERE d.department_id IS NULL;
+```
+
+---
+
+### **2.3 Find Departments Without Employees**
+```sql
+SELECT d.department_name
+FROM employees e
+RIGHT JOIN departments d
+ON e.department_id = d.department_id
+WHERE e.id IS NULL;
+```
+
+---
+
+### **2.4 Find Employees Working in Multiple Departments**
+```sql
+SELECT e.name, COUNT(d.department_id) AS department_count
+FROM employees e
+INNER JOIN departments d
+ON e.department_id = d.department_id
+GROUP BY e.name
+HAVING COUNT(d.department_id) > 1;
+```
+
+---
+
+### **2.5 Calculate Total Salary by Department**
+```sql
+SELECT d.department_name, SUM(e.salary) AS total_salary
+FROM employees e
+INNER JOIN departments d
+ON e.department_id = d.department_id
+GROUP BY d.department_name;
+```
+
+---
+
+### **2.6 Find Employees with Their Managers (Self Join)**
+```sql
+SELECT e1.name AS employee, e2.name AS manager
+FROM employees e1
+LEFT JOIN employees e2
+ON e1.manager_id = e2.id;
+```
+
+---
+
+## **3. Joining More Than Two Tables**
+
+```sql
+SELECT e.id, e.name, d.department_name, l.location
+FROM employees e
+INNER JOIN departments d
+ON e.department_id = d.department_id
+INNER JOIN locations l
+ON d.location_id = l.location_id;
+```
+**Use Case:**  
+- Combines **employees, departments, and locations**.
+
+---
+
+## **4. Nested Joins (Multiple JOIN Conditions)**
+
+```sql
+SELECT e.name, d.department_name, l.city
+FROM employees e
+LEFT JOIN departments d ON e.department_id = d.department_id
+LEFT JOIN locations l ON d.location_id = l.location_id
+WHERE l.city = 'Berlin';
+```
+
+**Use Case:**  
+- Find employees based in **Berlin**.
+
+---
+
+## **5. Tips for Joins**
+
+1. **Aliases:** Use aliases (**e, d**) to make queries more readable.  
+2. **Indexes:** Create indexes on **foreign keys** to improve performance.  
+3. **NULL Handling:** Always check for **NULL** values when using **OUTER JOINs**.  
+4. **Check Results:** Test with a **LIMIT 10** query before joining large datasets.  
+5. **Use Subqueries for Filtering:** Use subqueries to pre-filter data before joining, if performance slows down.
+
+---
+
+## **6. Real-World Use Cases**
+
+1. **Customer Orders Analysis:**
+```sql
+SELECT c.customer_name, o.order_date, p.product_name
+FROM customers c
+INNER JOIN orders o ON c.customer_id = o.customer_id
+INNER JOIN products p ON o.product_id = p.product_id;
+```
+
+2. **Top 3 Products by Region:**
+```sql
+SELECT r.region_name, p.product_name, SUM(s.sales) AS total_sales,
+       RANK() OVER (PARTITION BY r.region_name ORDER BY SUM(s.sales) DESC) AS rank
+FROM sales s
+INNER JOIN products p ON s.product_id = p.product_id
+INNER JOIN regions r ON s.region_id = r.region_id
+GROUP BY r.region_name, p.product_name;
+```
+
+3. **Employee Salary Growth Over Time:**
+```sql
+SELECT e.name, s.salary, s.salary - LAG(s.salary, 1) OVER (PARTITION BY e.id ORDER BY s.date) AS salary_growth
+FROM employees e
+INNER JOIN salaries s ON e.id = s.employee_id;
+```
+
+---
+
+CompliedByUdithaWick