PROC SQL allows a subquery (contained in parentheses) at any point in an expression where a simple column value or constant can be used. In this case, a subquery must return a single value, that is, ...

The outer PROC SQL query retrieves data from the view descriptor; the subquery uses a Pass-Through query to retrieve data. This query returns the names of employees who earn less than the average ...

Read this SQL tutorial to learn when to use SELECT, JOIN, subselects and UNION to access multiple tables with a single statement.