很难的SQL查询问题

--有15个同一年级(grade)学生(student)选修的课程的名字（cname）
select * from course where cid in
(select a.cid from temp a,course b,student c where a.cid=b.cid and a.sid=c.sid group by a.cid,c.grade having count(*)=15 )

--给所有年级都上过课的老师的名字(tname)
select * from teacher where tid in
(select a.tid from temp a,student b where a.sid=b.sid group by a.tid having count(distinct b.grade)=(select count(distinct grade) from student))

选修人数：
select grade,cid,count(*) as 选修人数
from student join temp on student.sid=temp.sid join course on temp.cid=course.cid join teacher on temp.tid=teacher.tid
group by grade,cid
having count(*)=15

全部课程都上过课的老师名字
select *
from temp join course on temp.cid=course.cid right join teacher on temp.tid=teacher.tid
where temp.tid is null

1\select c.cname
from course c,
(select a.grade,b.cid,count(*) cnt
from student a,temp b
where a.sid=b.sid
group by a.grade,b.