Lookup two objects list values in one object are not in the other object
I have two sources of reference numbers that ties our whole system together. Let's call them RefCodeSystemA and RefCodeSystemB. I would like to build a webi report that shows the Values in RefCodeSystemA that a NOT in RefCodeSystemB. I can easily find out the matching ones but not the NON matching onces?
Any suggestions on a simple query to do this?