找出有两门及两门以上不及格同学的平均分

select avg(score),name from stu where name =(select name from (select name,count(name) c from ( select  name ,score from stu where score<60) m group by name)

k  where c>=2) group by name;

//如果有两个同学满足条件就会有个bug，把=改成in就好了

另外增加一个显示挂科数的

select nn.ccc ,avg(score),stu.name from stu,(select count(score) ccc,name from stu where score<60 group by name) nn where stu.name in (select name from (select name,count(name) c from ( select  name ,score from stu where score<60) m group by name)  k  where c>=2) group by name;