Previous module: Module 7, How to define a metric clearly.
An INNER JOIN and a LEFT JOIN can use the same columns but return a different number of customers. The difference comes from what happens when a customer doesn’t have a matching order.
In this module, we’ll compare both joins with a customer coverage example. After that, we’ll see why a WHERE condition can still remove customers from a left join.
The first thing we need to understand is which table defines the population. If the report should include every customer, the result still needs eight customers after the join. If the report is only about customers with an order, unmatched customers don’t belong in the result.
