The Advanced Programming prep guide distinguishes correlated and non-correlated subqueries with the following definitions.    A noncorrelated subquery is a self-contained subquery that executes independently of the outer query. The simplest type of subquery is a noncorrelated subquery that returns a single value.   correlated -  A dependent subquery is one that requires one or more values to be passed to it by the outer query before the subquery can return a value to the outer query.   Later on, however, they mention the following SQL query being a correlated subquery but how can that be? How is the subquery in this case dependent on the outer query?   The following PROC SQL query accomplishes this task by using a correlated subquery and the NOT EXISTS operator. proc sql; select lastname, firstname from certadv.flightattendants where not exists (select * from certadv.flightschedule where flightattendants.empid= flightschedule.empid); quit;   ... View more