JOINs

What you will learn

How to combine data from multiple tables using JOIN.

INNER JOIN

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

Only rows where the condition matches appear in the result.

LEFT JOIN

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

All users appear, even those with no orders — total will be NULL.

Common mistakes

• Forgetting the ON clause — causes a cartesian product (every row × every row)
• Mixing up LEFT and INNER — test with a row that has no match to see the difference

Quick check below!