550. 游戏玩法分析 IV
题目链接：550. 游戏玩法分析 IV
代码如下：
# Write your MySQL query statement below
select
round(avg(a.event_date is not null),2)
as fraction
from
(select player_id,min(event_date) as login
from activity
group by player_id) as p
left join activity as a
on p.player_id=a.player_id and datediff(a.event_date,p.login)=1
