User Activity for the Past 30 Days I

Task: Find the daily active user count for a period of 30 days ending 2019-07-27 inclusive. A user was active on someday if they made at least one activity on that day.

Table: Activity

+---------------+-------------+
| Column Name   | Type        |
+---------------+-------------+
| user_id       | INT         |
| session_id    | INT         |
| activity_date | date        |
| activity_type | varchar(20) |
+---------------+-------------+

Input: 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   |
+---------+------------+---------------+---------------+

Output:

+------------+--------------+
| day        | active_users |
+------------+--------------+
| 2019-07-20 | 2            |
| 2019-07-21 | 2            |
+------------+--------------+