MySQL + SQL · Lesson 71

Nested Queries and Subqueries

What is a Subquery?

A subquery (nested query) is a query inside another query. The inner query runs first; its result feeds the outer query.

Subquery in WHERE

-- students scoring above the average
SELECT name, marks FROM students
WHERE marks > (SELECT AVG(marks) FROM students);

The inner query finds the average; the outer keeps students above it.

Subquery with IN

-- students who have paid fees
SELECT name FROM students
WHERE roll_no IN (SELECT roll_no FROM fees);

Summary

A subquery is a query inside another; the inner runs first.
Common with WHERE, IN and comparison operators.