SELECT S.SNO,S.SNAME,AVG_SCGRADE=AVG(SC.SCGRADE) FROM S,SC,( SELECT SNO FROM SC WHERE SCGRADE<60 GROUP BY SNO HAVING COUNT(DISTINCT CNO)>=2 )A WHERE S.SNO=A.SNO AND SC.SNO=A.SNO GROUP BY S.SNO,S.SNAME
3. 列出既学过“1”号课程,又学过“2”号课程的所有学生姓名
--实现代码: SELECT S.SNO,S.SNAME FROM S,( SELECT SC.SNO FROM SC,C WHERE SC.CNO=C.CNO AND C.CNAME IN('1','2') GROUP BY SNO HAVING COUNT(DISTINCT CNO)=2 )SC WHERE S.SNO=SC.SNO