1141. User Activity for the Past 30 Days I

https://leetcode.com/problems/user-activity-for-the-past-30-days-i

Description

Table: Activity

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| user\_id       | int     |
| session\_id    | int     |
| activity\_date | date    |
| activity\_type | enum    |
+---------------+---------+
There is no primary key for this table, it may have duplicate rows.
The activity\_type column is an ENUM of type ('open\_session', 'end\_session', 'scroll\_down', 'send\_message').
The table shows the user activities for a social media website. 
Note that each session belongs to exactly one user.

Write an SQL query to find the daily active user count for a period of 30 days ending 2019-07-27 inclusively. A user was active on some day if he/she made at least one activity on that day.

The query result format is in the following example:

Activity table:
+---------+------------+---------------+---------------+
| user\_id | session\_id | activity\_date | activity\_type |
+---------+------------+---------------+---------------+
| 1       | 1          | 2019-07-20    | open\_session  |
| 1       | 1          | 2019-07-20    | scroll\_down   |
| 1       | 1          | 2019-07-20    | end\_session   |
| 2       | 4          | 2019-07-20    | open\_session  |
| 2       | 4          | 2019-07-21    | send\_message  |
| 2       | 4          | 2019-07-21    | end\_session   |
| 3       | 2          | 2019-07-21    | open\_session  |
| 3       | 2          | 2019-07-21    | send\_message  |
| 3       | 2          | 2019-07-21    | end\_session   |
| 4       | 3          | 2019-06-25    | open\_session  |
| 4       | 3          | 2019-06-25    | end\_session   |
+---------+------------+---------------+---------------+

Result table:
+------------+--------------+ 
| day        | active\_users |
+------------+--------------+ 
| 2019-07-20 | 2            |
| 2019-07-21 | 2            |
+------------+--------------+ 
Note that we do not care about days with zero active users.

ac