MySQL Subqueries Tutorial: Queries Inside Queries

A subquery is a query nested inside another query. It lets you answer two-step questions in one statement — like 'find employees who earn more than the average salary' — without computing intermediate results by hand.

Key Points

• Scalar subquery returns one value: WHERE salary > (SELECT AVG(salary) FROM employees)
• IN subquery matches a list: WHERE dept_id IN (SELECT id FROM departments WHERE city='Noida')
• EXISTS checks whether matching rows exist — efficient for large sets
• Derived table: a subquery in FROM that acts like a temporary table
• Subqueries in SELECT compute per-row lookups (use carefully)
• Most subqueries can be rewritten as JOINs — know both forms