Linking Fields In Database Expert

I am using Crystal Reports 2008 and need help with the Database Expert. I have a transaction table with customer # and a master table with several customers. My question is I need to use my transaction table and compare against the master table using the field, customer #. If there isn't a match from the using the transaction table customer # with the customer # of the master file than I need to print the customer # from the transaction table that doesnt have a match. Not sure how to do this, any help would be greatly appreciated.

Thanks Jim Dyer