设教学数据库中有3个关系？

select TNO,TN from T where TN like  '_伟%'

select TN from T where TNO='C5'

select T.TNO,T.TN,TC.CNO from T,TC where T.TNO=TC.TNO and T.TN='刘伟'

select TC.TNO,group by TC.CNO as ‘任课数’from TC,T where TC.TNO=T.TNO

and T.Dept='计算机' and  T.TNO in (select TNO from TC group by CNO havingcount(*)>2)

selcet TNO,TN from T where Prof  in （select Prof from T where TN='刘伟'）

select x.TN as姓名,x.Sal as 教师工资, y.Sal as刘伟的工资from T as x,T as y

where x.Sal>y.Sal and y.TN='刘伟'

第四问不太确定

（1）检索计算机系的全体学生的学号，姓名和性别；

SELECT SNO,SNAME,SEX FROM S

WHERE SNO IN (

SELECT SNO FROM SC WHERE CNO IN (SELECT SNO FROM C WHERE CDEPT='计算机')

)

（2）检索学习课程号为C2的学生学号与姓名；

SELECT SNO,SNAME FROM S

WHERE SNO IN (SELECT SNO FROM SC WHERE CNO='C2')

（3）检索选修课程名为“DS”的学生学号与姓名；

SELECT SNO,SNAME FROM S

WHERE SNO IN (SELECT SNO FROM SC WHERE CNO IN (SELECT CNO FROM C WHERE CNAME='DS'))

（4）检索选修课程号为C2或C4的学生学号；

SELECT SNO FROM SC WHERE CNO IN ('C2','C4')

（5）检索至少选修课程号为C2和C4的学生学号；

SELECT SNO FROM S

WHERE SNO IN (SELECT SNO FROM SC WHERE CNO='C2')

AND SNO IN (SELECT SNO FROM SC WHERE CNO='C4')

（6）检索不学C2课的学生姓名和年龄；

SELECT SNAME,AGE FROM S

WHERE SNO NOT IN (SELECT SNO FROM SC WHERE CNO='C2')

AND SNO NOT IN (SELECT SNO FROM SC WHERE CNO='C4')

（7）检索学习全部课程的学生姓名；

SELECT SNAME FROM S

WHERE SNO IN (

SELECT SNO FROM SC WHERE NOT EXISTS (SELECT * FROM C WHERE SC.CNO=C.CNO)

)

（8）查询所学课程包含学生S3所学课程的学生学号。

SELECT SNAME FROM S

WHERE SNO IN (

SELECT SNO FROM SC WHERE NOT EXISTS (SELECT * FROM SC SC2 WHERE SC2.CNO=SC.CNO)

)

---