Subquery

Subqueries (also known as inner queries or nested queries) are a tool for performing operations in multiple steps. For example, if you wanted to take the sums of several columns, then average all of those values, you'd need to do each aggregation in a distinct step.

Subqueries can be used in several places within a query, but it's easiest to start with the FROM statement.

Subqueries are required to have names, which are added after parentheses the same way you would add an alias to a normal table.

A quick note on formatting: The important thing to remember when using subqueries is to provide some way for the reader to easily determine which parts of the query will be executed together. Most people do this by indenting the subquery in some way.