代码：
select class from a GROUP BY class HAVING COUNT(*)>=5
这样依旧会报错的，注意红色框里面，学生里是不可以重复的。
正确代码：
select a.class FROM (
select DISTINCT student ,class ,COUNT(*) from a GROUP BY class,student ) a GROUP BY a.class
HAVING COUNT(*)>=5
要防止学生重复
leetcode 提交运行截图
leetcode 代码:
select a.class FROM (
select DISTINCT student ,class ,COUNT(*) from courses GROUP BY class,student ) a GROUP BY a.class
HAVING COUNT(*)>=5