ABAP 'EXISTS' with variables

Hi ABAP, I need help on the following simple scenario:

• I want to count all the records in my TARGETTABLE that TARGETCOLUMN entries exist in SOURCECOLUMN in my SOURCE TABLE
• all of them have been defined as variables

SELECT COUNT( * )

  FROM (TARGETTABLE) INTO F1

  WHERE (TARGETCOLUMNCONDITION)

*Its working up to this point, but the following where clause does not work

    AND EXISTS ( SELECT *

                  FROM (TARGETTABLE)

                  WHERE (TARGETCOLUMN) = (SOURCETABLE)~(SOURCECOLUMN) ).

Error message: "Incorrect expression "=" in logical condition

Got the subquery from: https://help.sap.com/saphelp_nw04/helpdata/en/dc/dc7614099b11d295320000e8353423/content.htm

Thanks in advance!