1. List all stuids which are enrolled in ART 103A and which are enrolled in MTH 103C
Select C.STUID
FROM CLASS C
INNER JOIN ENROLL E ON C.CLASSID = E.CLASSID
WHERE C.CLASSID in ('ART103A', 'MTH103C')
2. List all stuids which are enrolled in ART 103A and which are not enrolled in MTH 103C
SELECT TMP.*
FROM
(
Select C.STUID
FROM CLASS C
INNER JOIN ENROLL E ON C.CLASSID = E.CLASSID
WHERE C.CLASSID = 'ART103A')
) TMP LEFT OUTER JOIN ENROLL EN ON TMP.STUID = EN.STUID AND EN.CLASSID = 'MTH103C'
WHERE EN.CLASSID IS NULL