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 someday if they made at least one activity on that day.

Return the result table in any order.

The query 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.