What is a subquery in PostgreSQL?

A subquery (inner query or nested query) is a query nested inside another query. It can appear in SELECT, FROM, WHERE, or HAVING clauses. Scalar subquery: returns one value — SELECT name, (SELECT COUNT(*) FROM orders WHERE user_id = u.id) AS order_count FROM users u;. Row subquery: returns one row — WHERE (col1, col2) = (SELECT a, b FROM ...). Table subquery (derived table): in FROM clause — SELECT * FROM (SELECT * FROM users WHERE active = true) AS active_users;. Correlated subquery: references outer query columns — re-executed for each row (often replaced by JOIN or CTE for performance). EXISTS, NOT EXISTS, IN, NOT IN, ANY, ALL operators work with subqueries.