1141. User Activity for the Past 30 Days I

Table: Activity

Column Name	Type
user_id	int
session_id	int
activity_date	date
activity_type	enum

This table may have duplicate rows.
The activity_type column is an ENUM (category) 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 a solution to find the daily active user count for a period of 30 days ending 2019-07-27 inclusively. A user was active on someday if they made at least one activity on that day.

Return the result table in any order.

The result format is in the following example.

Example 1:

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

Explanation: Note that we do not care about days with zero active users.

SELECT
    activity_date AS day,
    COUNT(DISTINCT user_id) AS active_users
FROM Activity
WHERE
    DATEDIFF('2019-07-27', activity_date) < 30
    AND
    activity_date <= '2019-07-27'
GROUP BY activity_date;