1学生表student(sno,sname,ssex,sage,sdept)Student由学号(Sno)姓名(Sname)性别(Ssex)年龄(Sage)所在系

1、SELECT A.sno,A.sname,A.ssex,A.sage,A.sdept,B.Cno,B.Cname,B.Cpno,B.Ccredit,C.Crede FROM student A JOIN Sc B ON A.sno=B.Sno JOIN Couse C ON B.Cno=C.Cno WHERE A.ssex='女生' AND A.sage=21

2、SELECT A.Cno,Cname,COUNT(Sno) AS 人数 FROM Couse A JOIN Sc B ON A.Cno=B.Cno GROUP BY A.Cno,Cname

---------------不知道你的成绩字段是不是Crede------------------
3、SELECT A.sno,A.sname,B.Cno,B.Cname,B.Cpno,B.Ccredit,C.Crede FROM student A JOIN Sc B ON A.sno=B.Sno JOIN Couse C ON B.Cno=C.Cno WHERE B.Cname='数据库' AND C.Crede=90

4、UPDATE student SET Sage=20 WHERE Sno='10240203'

5、"选秀1号课程" 是什么如果选修某个课程，参考3

6、DELETE FROM Sc WHERE Sno IN (SELECT Sno FROM student WHERE sdept='++系')

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