text: --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    |
  --+---------------+---------+
  --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.
  
  --# Write your MySQL query statement below
  select activity_date as day, count(distinct(user_id)) as active_users from Activity
  where activity_date between "2019-06-28" and "2019-07-27"
  group by activity_date;