SQL JOINs explained

A JOIN combines rows from two tables that share a related value — usually an ID. Without JOINs you can only look at one table at a time. With them you can answer questions like “which customers placed orders?” or “which employees have no projects?”

INNER JOIN returns only the rows that match in both tables. If a customer has no orders, they disappear from the result.

LEFT JOIN keeps every row from the left table and adds matching data from the right. Customers with no orders still appear, with empty order fields.

Most interview and work questions use these two. The exercises on this site walk you from a simple INNER JOIN through multi-table joins and LEFT JOIN edge cases.