Nested Count function and substring... Need some tips.tx

Question

I am trying to find out the number of active students who have the course 'MCB4U'
twice on their records.
Please let me a comment if you know the solution.
Thank you
Here is my code:
SELECT COUNT(*)
FROM PROG A, ENRL B
WHERE A.STUID = B.STUID
AND B.ENRL = 'E'
AND A.STATUS ='ACTIVE'
AND B.TERM = '2007'
     AND EXISTS (SELECT COUNT(E.SCHOOL_NBER)
                    FROM EXT_COURSE C, EXT_CRSE D, PS_SCHOOL_CRSE E
                              WHERE C.STUID = D.STUID
                              AND C.EXT_ID = D.EXT_ID
                              AND C.COURSE_NBR = D.COURSE_NBR
                              AND D.EXT_ID = E.EXT_ID
                              AND E.SCHOOL_SUBJECT = D.SCHOOL_SUBJECT
                              AND E.SCHOOL_NBR = D.SCHOOL__NBR
                              AND E.SCHOOL_NBR SUBSTR ('MCB4',5,1)
                              AND D.EMPLID = A.EMPLID
                              HAVING COUNT(*) > 2)
HAVING COUNT(*)=1 -- This statement identifies the distinct enroled students

Answer 1

I don't have your tables, but in general if you want "the number of active students who have the course 'MCB4U' twice on their records", then your query might look like this:
select count(*)
  from ( select count(*)
           from ...
          where ...
          group by stuid
         having count(*) = 2
       )Regards,
Rob.