where exists(
select sno,sum(credit) 'total credit' from SC join course c on SC.cno = c.cno group by SC.sno
having sum(credit) > 3)
17. 查询在第4学期开设课程中与第1学期开设的课程学分相同的课程，列出课
程名和学分。
select cname,credit from course
where semester in (1,4) and credit in(
select credit from course where semester = 4 intersect
select credit from course where semester = 1)
18. 查询“李勇”和“王大力”所选的相同的课程，列出课程名、开课学期和学
分。
select cname,semester,credit from course where cno in(
select cno from SC join student s on SC.sno = s.sno
where sname = '李勇' intersect
select cno from SC join student s on SC.sno = s.sno where sname = '王大力')
19. 查询“李勇”选了但“王大力”没有选的课程，列出课程名、开课学期和学
分。
select cname,semester,credit from course where Cno in(
select cno from SC join student s on SC.sno = s.sno where Sname = '李勇' except
select cno from SC join student s on SC.sno = s.sno where Sname = '王大力')
20. 查询至少同时选了“C001”和“C002”这两门课程的学生的学号和所选的课
程号。
select sno,cno from SC where sno in(
select sno from SC where cno = 'C001' intersect
select sno from SC where cno = 'C002')
*以上内容仅供参考