Simple MySQL Solution
Code


# Code
```mysql []
select round(count(distinct player_id)/(select count(distinct player_id) from Activity),2) as fraction
from Activity 
where (player_id,DATE_SUB(event_date,INTERVAL 1 DAY))
in(
select player_id,Min(event_date) as first_login
from Activity
group by player_id
)

```
['Database', 'MySQL']