What is a JOIN in SQL?

A JOIN combines rows from two or more tables based on a related column. Types: INNER JOIN — returns only rows with matching values in both tables; LEFT (OUTER) JOIN — returns all rows from the left table and matched rows from the right (NULLs for no match); RIGHT (OUTER) JOIN — all rows from the right, matched from left; FULL OUTER JOIN — all rows from both tables; CROSS JOIN — Cartesian product (every row of A × every row of B); SELF JOIN — a table joined to itself (e.g., employees and their managers in the same table). Always specify JOIN conditions to avoid accidental Cartesian products.