SQL · Chapter 15 of 42

SQL JOIN Overview

JOIN combines rows from two or more tables based on a related column — usually a foreign key.

Main types: INNER, LEFT, RIGHT, FULL OUTER, and CROSS.

Syntax

SELECT ... FROM a JOIN b ON a.b_id = b.id;

Types at a glance

INNER: only matching rows. LEFT: all from left + matches from right. RIGHT: mirror of LEFT. FULL: all from both. CROSS: cartesian product.

The `ON` clause states the matching condition, usually `left.fk = right.pk`.

Example 1 (sql)

SELECT u.name, o.total
FROM users u
INNER JOIN orders o ON o.user_id = u.id;

Output

one row per matching (user, order) pair

Classic parent-child join.

Example 2 (sql)

SELECT u.name, o.total
FROM users u
LEFT JOIN orders o ON o.user_id = u.id;

Output

all users, orders where present, NULL otherwise

LEFT keeps every user.

💡 Note: Almost every JOIN you'll write in real projects is INNER or LEFT. RIGHT and FULL are rare.