经典SQL题6

问题描述：
已知关系模式：
S (SNO,SNAME）          学生关系。SNO 为学号，SNAME 为姓名
C (CNO,CNAME,CTEACHER)  课程关系。CNO 为课程号，CNAME 为课程名，CTEACHER 为任课教师
SC(SNO,CNO,SCGRADE)     选课关系。SCGRADE 为成绩

1. 找出没有选修过“李明”老师讲授课程的所有学生姓名
2. 列出有二门以上（含两门）不及格课程的学生姓名及其平均成绩
3. 列出既学过“1”号课程，又学过“2”号课程的所有学生姓名
4. 列出“1”号课成绩比“2”号同学该门课成绩高的所有学生的学号
5. 列出“1”号课成绩比“2”号课成绩高的所有学生的学号及其“1”号课和“2”号课的成绩

1:
SELECT SNAME
FROM S
WHERE SNO NOT IN
(SELECT SNO FROM SC WHERE CNO IN(SELECT CNO FROM C WHERE CTEACHER = '李明'))

2:
SELECT S.SNO,S.SNAME,平均成绩=AVG(SC.SCGRADE)
FROM S,SC,(SELECT SNO FROM SC WHERE SCGRADE < 60 GROUP BY SNO HAVING COUNT(DISTINCT CNO)>=2) as A
WHERE S.SNO=A.SNO AND SC.SNO=A.SNO
GROUP BY S.SNO,S.SNAME

3:
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 SC.SNO
    HAVING COUNT(DISTINCT SC.CNO)=2) SC
WHERE S.SNO=SC.SNO

4:
SELECT S.SNO,S.SNAME
FROM S,SC AS SC1,SC AS SC2
    WHERE SC1.CNO='1'
        AND SC2.SNO='2'
        AND SC1.CNO=SC2.CNO
        AND SC1.SCGRADE > SC2.SCGRADE

5:
SELECT SC1.SNO,[1号课成绩]=SC1.SCGRADE,[2号课成绩]=SC2.SCGRADE
FROM SC SC1,SC SC2
WHERE SC1.CNO='1'
        AND SC2.CNO='2'
        AND SC1.SNO= SC2.SNO
        AND SC1.SCGRADE > SC2.SCGRADE