sql24（Leetcode1141查询近30天活跃用户数）

代码：

# Write your MySQL query statement below

select v.activity_date as day, count(distinct(v.user_id)) as active_users
from(
    select user_id,activity_date
    from Activity
    where activity_date between '2019-06-28' and '2019-07-27'
) as v
group by v.activity_date