數據庫題目6

題目描述：
已知關係模式：
S (SNO,SNAME) 學生關係。SNO爲學號，SNAME爲姓名；
C (CNO,CNAME,CTEACHER) 課程關係。CNO爲課程號，CNAME爲課程名，CTEACHER爲任課教師；
SC(SNO,CNO,SCGRADE)選課關係。SCGRADE爲成績。
問題：
1) 找出沒有選修過“李明”老師講授課程的所有學生姓名；
SELECT SNAME
FROM S WHERE
NOT EXISTS (SELECT *
FROM C, SC
WHERE SC.CNO = C.CNO
AND CTEACHER = ‘李明’);

2) 列出有二門以上(含兩門)不及格課程的學生姓名及其平均成績；
SELECT SNAME, AVG(SCGRADE)
FROM S, SC
WHERE S.SNO = SC.SNO
AND SNO
IN (SELECT SNO
FROM SC
WHERE SCGRADE < 60
GROUP BY SNO
HAVING COUNT(*) >= 2);

3) 列出既學過課程名爲“1”的課程，又學過課程名爲“2”的課程的所有學生姓名；
SELECT 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) K
WHERE S.SNO=K.SNO;

4) 列出“1”號課成績比“2”號課成績高的所有學生的學號及其“1”號課和“2”號課的成績。
SELECT S.SNO, T.GRADE1, T.GRADE2
FROM S,
(SELECT SC1.SNO, GRADE1 = SC1.SGRADE, GRADE2 = SC2.SGRADE
FROM SC SC1, C C1, SC SC2, C C2
WHERE SC1.SCGRADE > SC2.SCGRADE
AND SC1.CNO = C1.CNO
AND C1.CNAME = ‘1’
AND SC2.CNO = C2.CNO
AND C2.CNAME = ‘2’ ) T
WHERE S.SNO = T.SNO;