MySql 1596. Class with more than 5 students

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1. Title description

2. Answer ideas

> 首先根据课程进行分组
> 但是有个问题就是有可能出现学生重复选课
> 所以分组后要对其去重输出
> 或者在分组前将其去重之后再分组

3. MySql language

select class
from courses
group by class
having count(distinct student)>=5