LeetCode MySQL 1107. 每日新用户统计

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| user_id       | int     |
| activity      | enum    |
| activity_date | date    |
+---------------+---------+
该表没有主键，它可能有重复的行。
activity 列是 ENUM 类型，
可能取 ('login', 'logout', 'jobs', 'groups', 'homepage') 几个值之一。

Traffic 表：
+---------+----------+---------------+
| user_id | activity | activity_date |
+---------+----------+---------------+
| 1       | login    | 2019-05-01    |
| 1       | homepage | 2019-05-01    |
| 1       | logout   | 2019-05-01    |
| 2       | login    | 2019-06-21    |
| 2       | logout   | 2019-06-21    |
| 3       | login    | 2019-01-01    |
| 3       | jobs     | 2019-01-01    |
| 3       | logout   | 2019-01-01    |
| 4       | login    | 2019-06-21    |
| 4       | groups   | 2019-06-21    |
| 4       | logout   | 2019-06-21    |
| 5       | login    | 2019-03-01    |
| 5       | logout   | 2019-03-01    |
| 5       | login    | 2019-06-21    |
| 5       | logout   | 2019-06-21    |
+---------+----------+---------------+

Result 表：
+------------+-------------+
| login_date | user_count  |
+------------+-------------+
| 2019-05-01 | 1           |
| 2019-06-21 | 2           |
+------------+-------------+
请注意，我们只关心用户数非零的日期.
ID 为 5 的用户第一次登陆于 2019-03-01，
因此他不算在 2019-06-21 的的统计内。

# Write your MySQL query statement below
select login_date, count(*) user_count
from
(
    select user_id, min(activity_date) login_date
    from Traffic
    where activity = 'login'
    group by user_id
) t
where datediff('2019-06-30', login_date) <= 90
group by login_date