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 name of a PROC SQL table. the name of a SAS data view. a query-expression. A query-expression in the FROM clause is usually referred to as an in-line view. See FROM Clause for more information on ...