LEFT JOIN.

LEFT JOIN keeps every left-side row. Unmatched right-side columns become NULL. This includes customers without orders. A WHERE condition on the right side can accidentally remove those rows.

Example: SELECT c.name, o.id AS order_id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id lists every customer; without an order, order_id is NULL. That differs from INNER JOIN, which drops unmatched customers.

Small example:

SELECT c.id, o.id AS order_id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id;