题目内容
(请给出正确答案)
[单选题]
设选修C1课程的学生关系为R,选修C2课程的学生关系为S,求同时选修了C1和C2的学生,则需进行的运算是______。
A.并
B.交
C.差
D.或
答案
(1)SELECT SNAME,AGEFROM S,SC,CWHERE S.S#=SC.S#AND C.C#=SC.C#AND CNAME=’ MATHS’(2) SELECT S# FROM SCWHERE CNO=’C1’ AND S# IN(SELECT S# FROM SCWHERE CNO=’C2’)(3)SELECT S#,SNAME,AGE FROM SWHERE AGE BETWEEN 18 AND 20 AND SEX=“女”(4) SELECT S#,AVG(GRADE) ‘平均成绩’FROM SCGROUP BY S# HAVING AVG(GRADE)>80(5) SELECT SNAME FROM SWHERE NOT EXISTS(SELECT * FROM CWHERE NOT EXISTS(SELECT * FROM SCWHERE S#=S.S#AND C#=C.C#))(6) SELECT SNAME FROM S,SCWHERE S.S#=SC.S#GROUP BY S# HAVING COUNT(*)>3
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