[MySQL] simple solution, 100 %
Idea The idea is easy, for each player_id, find its first login date, and then decide whether the next date is in the event_date set, which can be achieved by j
## Idea 
The idea is easy, for each `player_id`, find its first login date, and then decide whether the next date is in the `event_date` set, which can be achieved by joining table `activity` with itself.  


## MYSQL solution

```sql
SELECT ROUND( COUNT(t2.player_id) / COUNT(t1.player_id), 2 ) AS fraction
FROM (SELECT DISTINCT player_id
    , MIN(event_date) AS first_day
FROM activity
    GROUP BY player_id ) t1
    LEFT JOIN activity t2 ON t1.player_id = t2.player_id AND t1.first_day + 1 = t2.event_date
```
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