How to use a parameter derived from a Table, given I'm filtering the table in query

Question

The task is to find what fraction of total users have logged in at least for one more consecutive day after their first login.

SELECT 
    ROUND( COUNT( b.player_id )/COUNT( DISTINCT (SELECT a.player_id FROM Activity a) ), 2 ) AS fraction
FROM 
    Activity b
WHERE 
    (b.player_id, DATE_ADD(b.event_date, INTERVAL -1 DAY)) IN
(
    SELECT 
        player_id, MIN(event_date)
    FROM 
        Activity
    GROUP BY 
        player_id
)```

The table structure is as follows

Table: Activity

+--------------+---------+
| Column Name  | Type    |
+--------------+---------+
| player_id    | int     |
| device_id    | int     |
| event_date   | date    |
| games_played | int     |
+--------------+---------+

Activity table:
+-----------+-----------+------------+--------------+
| player_id | device_id | event_date | games_played |
+-----------+-----------+------------+--------------+
| 1         | 2         | 2016-03-01 | 5            |
| 1         | 2         | 2016-03-02 | 6            |
| 2         | 3         | 2017-06-25 | 1            |
| 3         | 1         | 2016-03-02 | 0            |
| 3         | 4         | 2018-07-03 | 5            |
+-----------+-----------+------------+--------------+

I'm using two aliases a and b for the same table for referring them.

• a is for the unfiltered table, because I need a count of all Distinct users.
• b is for the filtered table, where only those rows are included for which a condition is met.

Where am I doing it wrong?