有student，course，choose_course 三张表，找出选了所有课程的学生名单

 解法1：
select sname from student s
where id in(select sid from choose_course
group by sid having count(sid)=(select count(1) from course))
解法2：
select distinct sname from student s
where not exists(
select * from course c
where not exists(
select * from choose_course cc
where cc.cid=c.id and cc.sid=s.id))