LeetCode 1142.过去30天的用户活动2

数据准备

drop table Activity;
Create table If Not Exists Activity (user_id int, session_id int, activity_date date, activity_type ENUM('open_session', 'end_session', 'scroll_down', 'send_message'));
Truncate table Activity;
insert into Activity (user_id, session_id, activity_date, activity_type) values ('1', '1', '2019-07-20', 'open_session');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('1', '1', '2019-07-20', 'scroll_down');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('1', '1', '2019-07-20', 'end_session');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('2', '4', '2019-07-20', 'open_session');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('2', '4', '2019-07-21', 'send_message');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('2', '4', '2019-07-21', 'end_session');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('3', '2', '2019-07-21', 'open_session');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('3', '2', '2019-07-21', 'send_message');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('3', '2', '2019-07-21', 'end_session');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('3', '5', '2019-07-21', 'open_session');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('3', '5', '2019-07-21', 'scroll_down');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('3', '5', '2019-07-21', 'end_session');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('4', '3', '2019-06-25', 'open_session');
insert into Activity (user_id, session_id, activity_date, activity_type) values ('4', '3', '2019-06-25', 'end_session');

需求

编写 SQL 查询以查找截至 2019-07-27（含）的 30 天内每个用户的平均会话数，四舍五入到小数点后两位。只统计那些会话期间用户至少进行一项活动的有效会话。

输入

输出

with t1 as (
    select user_id,session_id
    from Activity
    where activity_date<='2019-07-27' and activity_date>date_sub('2019-07-27',interval 30 day )
    group by user_id,session_id
),t2 as (
    select user_id,count(1) as cnt1
    from t1
    group by user_id
)
select round(sum(cnt1)/count(user_id),2) as average_sessions_per_user
from t2;