Getting the max count for each group

Question

Hi, I am a newbie and working on a project for school. I have been working this problem for a couple of days and made a bit of progress.
I have two tables, student and enroll for a college database. I need to show the students who have the most enrollments in each major. So far, I have:
WITH COUNT_Q AS
SELECT B.SSN, B.MAJOR, COUNT (A.CLASS_NO) AS COUNTCLASS
FROM ENROLL A, STUDENT B
WHERE A.SSN = B.SSN
GROUP BY B.SSN, B.MAJOR
SELECT MAX(COUNTCLASS), MAJOR
FROM COUNT_Q
GROUP BY MAJOR
I am getting the correct results but need to show the SSN and Major of the studens. When I add in the SSN and Major, like below, I get back too many records.
WITH COUNT_Q AS
SELECT B.NAME, B.SSN, B.MAJOR, COUNT(A.SSN) AS COUNTCLASS
FROM ENROLL A, STUDENT B
WHERE A.SSN = B.SSN
GROUP BY B.NAME, B.SSN, B.MAJOR
SELECT *
FROM COUNT_Q
WHERE (COUNTCLASS) IN (SELECT MAX(COUNTCLASS)
FROM COUNT_Q
GROUP BY MAJOR
Can someone help point me in the right direction?

Answer 1

SELECT  B.NAME,
        B.SSN,
        B.MAJOR
  FROM  (
         SELECT  B.NAME,
                 B.SSN,
                 B.MAJOR,
                 COUNT(A.CLASS_NO) SSN_MAJOR_COUNTCLASS
                 DENSE_RANK() OVER(PARTITION BY MAJOR ORDER BY COUNT(A.CLASS_NO) DESC) RNK
           FROM  ENROLL A,
                 STUDENT B
           WHERE A.SSN = B.SSN
           GROUP BY B.NAME,
                    B.SSN,
                    B.MAJOR
  WHERE RNK = 1
/SY.
Edited by: Solomon Yakobson on Mar 27, 2011 6:53 PM