What is a subquery in SQL?

Subqueries let you break a complex question into a smaller, self-contained query first, then use its result inside a larger one.

Example

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

Here, the subquery (SELECT AVG(salary) FROM employees) runs first, calculating the average salary. The outer query then uses that single value to filter for employees earning above it.

Common types of subqueries

• Scalar subquery: returns a single value, usable anywhere a single value is expected (like the example above)
• Row/column subquery: returns a set of values, often used with IN or EXISTS
• Correlated subquery: references a column from the outer query, so it re-runs once for each row the outer query processes — powerful, but can be slower than an equivalent JOIN for large datasets

Many subqueries can be rewritten as JOINs and vice versa — the right choice usually comes down to which is clearer to read and which performs better for a specific database and dataset.