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class(id,name)
stu(id,name,classid)
select class.id,class.name,count(stu.id) from class join stu on class.id=stu.classid
group by class.id order by count(stu.id) desc
班级id 班级name
1 name1
2 name2
学生id name classid
1 name1 1
2 name2 1
3 name3 2
4 name4 2
5 name5 2
id name count(stu.id)
2 name2 3
1 name1 2