查询存在两门（包含两门）以上课程不及格的学生姓名

刚正好在做这题，我这边是ok的。你自己可以去试试。（因为是测试，所以起名有些随意）
-- 查询所有有两门/或以上成绩小于60的学生姓名和平均成绩
SELECT s.sname,sss.mm from
(SELECT sc.sno,avg(sc.score) mm FROM sc WHERE sc.sno IN
(SELECT sc.sno FROM sc WHERE sc.score <60 GROUP BY sc.sno HAVING COUNT(sno)>1 )
GROUP BY sc.sno) sss
LEFT JOIN s on sss.sno = s.sno
解释：(SELECT sc.sno FROM sc WHERE sc.score <60 GROUP BY sc.sno HAVING COUNT(sno)>1 ) ——————是用来锁定包含两门不及格的学生的学号，并作为下面查询的约束条件
(SELECT sc.sno,avg(sc.score) mm FROM sc WHERE sc.sno IN ——————结合上面筛选出来的学号，计算出平均成绩，因为使用了avg函数所以需要排序GROUP BY sc.sno) sss，将中间表取名sss
LEFT JOIN s on sss.sno = s.sno——————用外链接关联学生信息表

select * from student where sno in(
select sno from sc where score<60 group by sno having count(cno)>=2)

select student.sname from student,sc
where score <60