1、题目描述
We define the install date of a player to be the first login day of that player.
We also define day 1 retention of some date X to be the number of players whose install date is X and they logged back in on the day right after X, divided by the number of players whose install date is X, rounded to 2 decimal places.
Write an SQL query that reports for each install date, the number of players that installed the game on that day and the day 1 retention.
The query result format is in the following example:
Activity table:
| player_id | device_id | event_date | games_played |
|---|---|---|---|
| 1 | 2 | 2016-03-01 | 5 |
| 1 | 2 | 2016-03-02 | 6 |
| 2 | 3 | 2017-06-25 | 1 |
| 3 | 1 | 2016-03-01 | 0 |
| 3 | 4 | 2016-07-03 | 5 |
Result table:
| install_dt | installs | Day1_retention |
|---|---|---|
| 2016-03-01 | 2 | 0.50 |
| 2017-06-25 | 1 | 0.00 |
Player 1 and 3 installed the game on 2016-03-01 but only player 1 logged back in on 2016-03-02 so the day 1 retention of 2016-03-01 is 1 / 2 = 0.50
Player 2 installed the game on 2017-06-25 but didn’t log back in on 2017-06-26 so the day 1 retention of 2017-06-25 is 0 / 1 = 0.00
来源：力扣（LeetCode）
2、解题思路
这一题，难度在于两个比较值‘安装日期’和‘第1天留存’，‘安装日期’可以直接按日期统计，但是‘第1天留存’必须先按玩家统计，然后转换成日期统计。
1# 首先‘安装’日期比较简单，直接进行count(player_id) as installs，但是注意要先进行排序order by player_id,event_date
select event_date as install_dt,count(player_id) as installs
from
(select player_id,event_date
from Activity
group by player_id
order by player_id,event_date)a
group by event_date
2# ‘第1天留存’，两表联查，取日期差为1，datediff(a2.event_date,a1.event_date)=1
select a1.event_date,count(a1.player_id) as jude
from Activity a1,Activity a2
where a1.player_id=a2.player_id and datediff(a2.event_date,a1.event_date)=1
group by a1.event_date
3# 2表联查，注意空置处理ifnull(jude,0)
3、提交记录
select install_dt,installs,round(ifnull(jude,0)/installs,2) as Day1_retention
from (
select event_date as install_dt,count(player_id) as installs
from
(select player_id,event_date
from Activity
group by player_id
order by player_id,event_date)a
group by event_date)c
left join
(select a1.event_date,count(a1.player_id) as jude
from Activity a1,Activity a2
where a1.player_id=a2.player_id and datediff(a2.event_date,a1.event_date)=1
group by a1.event_date)b
on c.install_dt=b.event_date
group by install_dt
order by install_dt
