select sname from student where sid in(
select sid from mark  group by sid having max(cmark)<80)

select sname from student where sid in(
select sid from mark where sid not in(
select sid from mark where cmark>=80))

Step1:select sid from student where sname=‘张三‘
step2：select cid from mark where sid=(step1)
step3:select sid from mark where cid not in (step2)
step4:select sid from mark where sid not in(step3)group by sid having count(*)=(select count(*)from mark where sid=(step1))

select sname ,(
select cmark from mark where sid=(select sid from student where sname=‘张三‘) 
and 
 cid=(select cid from course where cname=‘数学‘)
) 数学成绩,
(
select cmark from mark where sid=(select sid from student where sname=‘张三‘) 
and 
 cid=(select cid from course where cname=‘英语‘)
) 英语成绩 from studen

至少选修了学生10002选修的全部课程的学生学号

m studenm nt s leftjoin mark m on s.sid=m.sid where cmark<60

select sname from studenm nt s left join mark m on s.sid=m.sid where cmark<60

select sid from mark sc1 where not exists(
select * from mark sc2 where sid=10002 and not exists
(select *from mark where sid=sc1.sid and cid=sc2.cid
)
) and sid !=10002 group by sid

select sid from mark sc1 where not exists(
select * from mark sc2 where sid=(select sid from student where sname=‘张三‘) and not exists
(select *from mark where sid=sc1.sid and cid=sc2.cid
)
) and sid !=10002 group by sid