思路1

总体思路：用子查询分别求出 “第一天登录后的一天登录过的玩家的数量” 和 “总玩家数量”

先求所有玩家第一天登录的日期

SELECT player_id, MIN(event_date) FROM Activity GROUP BY player_id;

在用这个日期和DATEDIFF，求出第一天登录后的一天登录过的玩家的数量

SELECT COUNT(DISTINCT a.player_id)
FROM Activity a JOIN 
(SELECT player_id, MIN(event_date) AS event_date FROM Activity GROUP BY player_id) b
ON a.player_id=b.player_id AND DATEDIFF(a.event_date, b.event_date)=1;

总玩家数量

SELECT COUNT(DISTINCT player_id)
FROM Activity;

最后ROUND来除就行

SELECT ROUND(
(
SELECT COUNT(DISTINCT a.player_id)
FROM Activity AS a JOIN
(SELECT player_id, MIN(event_date) as event_date
FROM Activity
GROUP BY player_id) AS b
ON
    a.player_id=b.player_id AND
    DATEDIFF(a.event_date, b.event_date)=1
) /
(
SELECT COUNT(DISTINCT player_id)
FROM Activity
), 2) AS fraction;