WITH
Players AS (
SELECT player_id, MIN(event_date) AS first_login
FROM Activity
GROUP BY 1
)
SELECT ROUND(
COUNT(Players.player_id) / (
SELECT COUNT(DISTINCT Activity.player_id)
FROM Activity
),
2
) AS fraction
FROM Players
INNER JOIN Activity
ON (
Players.player_id = Activity.player_id
AND DATEDIFF(Players.first_login, Activity.event_date) = -1)