Previous module: Module 8, Choosing between INNER JOIN and LEFT JOIN.
An order total can be correct in the orders table but become wrong after a join. This happens when one order matches several item rows. SQL repeats the order value on every matched row, and SUM counts every copy.
In this module, we’ll learn how to check the row count after a join. We will also aggregate each child table before joining when the report needs one row per order.
