Very simple MYSQL solution
\nSELECT ROUND(COUNT(t2.player_id)/COUNT(t1.player_id),2) AS fraction\nFROM\n(SELECT player_id, MIN(event_date) AS first_login FROM Activity GROUP BY player_id)
```\nSELECT ROUND(COUNT(t2.player_id)/COUNT(t1.player_id),2) AS fraction\nFROM\n(SELECT player_id, MIN(event_date) AS first_login FROM Activity GROUP BY player_id) t1 LEFT JOIN Activity t2\nON t1.player_id = t2.player_id AND t1.first_login = t2.event_date - 1\n```
[]