Looking at your formula, I assume that customers is your master table and dist_customer_billing_profiles is the child. If this is correct, you need a left outer join FROM customer TO dist_customer_billing_profiles. Then you should change your formula to this:
If IsNull({dist_customer_billing_profiles.customer_no}) then "COPS" else "DI"
Note that you do NOT need the variable. The reason that your original formula does not work has to do with the way Null works. Null means "no value". When you do a comparison of the values of two fields and one or both is null, the result is null - NOT true or false. For a good explanation of of how null works, go to the notes on the SAP Crystal Reports and Dashboard Design Facebook page (I can't get to Facebook from work or I would send you the link) - Last summer I wrote a note on "What is Null and Why is it Important" that is posted here.
-Dell Edited by hilfy - 25 Apr 2011 at 8:06am