SQL · Chapter 16 of 42

INNER JOIN

INNER JOIN returns rows where the join condition matches in BOTH tables. Non-matching rows are excluded.

Implicit vs explicit

Old style: `FROM a, b WHERE a.id = b.a_id`. Modern: `FROM a JOIN b ON a.id = b.a_id`. Always use the modern form.

Chain multiple JOINs left-to-right; each ON refers to already-joined tables.

Example 1 (sql)

SELECT u.name, o.total
FROM users u
INNER JOIN orders o ON o.user_id = u.id
WHERE o.total > 500;

Output

users with big orders

Join + filter.

Example 2 (sql)

SELECT o.id, u.name, p.name
FROM orders o
JOIN users u ON u.id = o.user_id
JOIN products p ON p.id = o.product_id;

Output

orders enriched with user and product

Three-way join.

💡 Note: `JOIN` without a prefix defaults to `INNER JOIN` in every mainstream database.