🔎 SQL Simple & Correlated Subqueries Tutorial

🔍 SQL Tutorial: Simple and Correlated Subqueries

Subqueries are queries inside other queries. They help you fetch data based on complex conditions.

📋 Sample employees Table:

employee_id	name	department_id	salary
1	Arif	101	50000
2	Nasrin	102	60000
3	Rafi	101	55000
4	Ritu	103	45000
5	Faruk	102	62000

1️⃣ Simple Subquery

Find employees who earn more than the average salary of all employees.

SELECT name, salary
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);

Output:

name	salary
Nasrin	60000
Faruk	62000
Rafi	55000

2️⃣ Correlated Subquery

Find employees whose salary is greater than the average salary of their own department.

SELECT e1.name, e1.salary, e1.department_id
FROM employees e1
WHERE e1.salary > (
  SELECT AVG(e2.salary)
  FROM employees e2
  WHERE e2.department_id = e1.department_id
);

Output:

name	salary	department_id
Rafi	55000	101
Nasrin	60000	102
Faruk	62000	102

📌 Summary:

• Simple Subquery: Independent query inside WHERE clause.
• Correlated Subquery: Subquery depends on outer query data.
• Correlated subqueries execute once per row of outer query.

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