Q:

Not a GROUP BY Expression, but why? 

I'm completely confused as to why this code is not a GROUP BY expression:
SELECT Section_ID, COUNT(Student_ID) 
FROM Enrollment
WHERE Enroll_Date = TO_DATE('22-FEB-2007', 'DD-MON-YYYY')
GROUP BY Section_ID
ORDER BY Section_ID DESC, Student_ID;

I know it has something to do with the COUNT function, but I can't figure out why.  Even if I add COUNT(Student_ID) to the GROUP BY I get a different error saying group function isn't allowed here.  What am I doing wrong? 

A:

Studentid is not now part of the result, so you cannot order by that.  You could order by the count values. 
Note also.  By grouping on section_id there is only one unique row for each of those, so ordering by a second column isn't needed.