Chris Webb introduces the subselect in this short 3-min video. Log-in or get a free account to watch it!
The purpose of subselects in MDX is often misunderstood and they are frequently used incorrectly. A query with a subselect has an MDX SELECT statement in its FROM clause, instead of the name of a cube. It allows you to pre-filter the members on a hierarchy before you run a query. The main reason you would want to do this is to calculate subtotal values—a subselect runs a query in a what-if scenario, as if a hierarchy only contained some of its members and not all of them.
You should not confuse a subselect with the WHERE clause because, although they return the same results in some cases, they differ in several important respects. While this short video introduces the concept of a subselect, make sure to watch Chris’s full-length module on Exists, MDX Solve Order, and Subselects to learn about those important differences.
