1141. User Activity for the Past 30 Days I mysql

https://leetcode.com/problems/user-activity-for-the-past-30-days-i/?envType=study-plan&id=sql-i 

User Activity for the Past 30 Days I - LeetCode

select activity_date as day, count(distinct user_id) as active_users from activity where datediff('2019-07-27', activity_date) < 30 and datediff('2019-07-27', activity_date) >= 0 group by activity_date;