Jay,

Here's a fairly good example for using subselects.

Background:

Employee table one rec of each person

Training table one rec for each training program for each person

employees are "on probation" until they complete their orientation and OSHA training programs

Question:
create a list of employees that are NOT on probation

Answer:
There is only one way to get the answer and that is;

SELECT Employee.* ;
  FROM Employee JOIN Training ;
    ON Employee.EmpId = Training.EmpId ;
 WHERE Training.Program = "Orientation" ;
   AND Employee.EmpId IN ;
       (SELECT EmpId ; 
          FROM Training ;
         WHERE Program = "OSHA") ...