JOIN

What it is

JOIN combines rows using related keys.

When to use it

Read related data without per-row queries.

Syntax

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

Example

SELECT u.id, u.name, COALESCE(SUM(o.total_cents), 0) AS paid_cents
FROM users u
LEFT JOIN orders o ON o.user_id = u.id AND o.status = 'paid'
GROUP BY u.id, u.name;

Common mistakes

Filtering right-hand rows in WHERE can turn a LEFT JOIN into an inner join; joins can multiply totals.

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