Leetcode 다시풀기 1141 dau

1141. User Activity for the Past 30 Days I

데이터 설명 중 

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.

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')
      AND activity_type IS NOT NULL 
GROUP BY activity_date

매번 풀던 거라 코드는 매우 간단하게 풀었다. 

다른 풀이 

select activity_date as day, count(distinct user_id) as active_users 
from Activity
where datediff('2019-07-27', activity_date) <30
group by activity_date