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 ...

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, ...

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