https://leetcode.com/problems/duplicate-emails/description/
首先sql的执行顺序是
from-->where-->group by--->having-->select-->order by
所以用了group后，用where判断是错误的
# Write your MySQL query statement below
select distinct Email //这里不需要distinct，因为group后，相同的只算一个
from Person
group by Person.Email
having count(Email) >= 2;
https://leetcode.com/problems/classes-more-than-5-students/description/
group by之后，所有你group了的东西会放在一起。只算一个
不会输出多次
# Write your MySQL query statement below
select class
from courses
group by class
having count(DISTINCT student) >= 5;