Subqueries

What it is

A subquery nests a query inside another query.

When to use it

Use EXISTS for presence without multiplying rows.

Syntax

SELECT u.id FROM users u WHERE EXISTS (SELECT 1 FROM orders o WHERE o.user_id = u.id);

Example

SELECT u.id, u.name FROM users u
WHERE EXISTS (
    SELECT 1 FROM orders o
    WHERE o.user_id = u.id AND o.status = 'pending'
);

Common mistakes

NOT IN with NULL can surprise; consider NOT EXISTS and inspect query plans.

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