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, ...

from product as p --using join select p.pid, p.pname, count (o.productid)as Total_Order from product as p inner join ProductOrder as o on p.pid=o.productId group by p.pname,p.pid -- sub Query to ...

A nested subquery is a query inside another SQL statement that does not reference any column from the outer query. A correlated subquery is a subquery that references columns from the outer query.