java面试中关于学员一些sql语句的写法

1：查询选修课程为computer的学员名字

select name from stutents where sid in(select sid from chengji,kecheng where chengji.kid=kecheng.kid and kecheng.name='comput') 

2：查询没有选修课程为c#的学员名字

select name from stutents where sid not in( select sid from chengji,kecheng where chengji.kid=kecheng.kid and kecheng.name='c#') 

3：查询选修了所有课程的学员名字

select name from stutents where sid in (select sid from chengji right join kecheng on chengji.kid=kecheng.kid group by sid having count(*)=(select count(*) from kecheng)) 

4：查询选修了课程的学员名字

select 人数=count(distinct sid) from chengji 

5：查询选修课程最少3门的学员名字

select name from stutents where sid in( select sid from chengji group by sid having count(distinct kid)>=3)