SELECT
Activity.player_id,
Activity.event_date,
SUM(PrevActivity.games_played) AS games_played_so_far
FROM Activity
INNER JOIN Activity AS PrevActivity
ON (
Activity.player_id = PrevActivity.player_id
AND Activity.event_date >= PrevActivity.event_date)
GROUP BY 1, 2
ORDER BY 1, 2;
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