Probably an easy one...I have two tables, each with a number field say "Cat-ID" and "Ref-ID". The query I am looking to write will show the outstanding values. For example
Cat-ID: 100, 200, 300, 400, 500
Ref-ID: 100, 200, 500
Query Result: 300, 400
Any ideas on how to do this? I'm stumped.