Solution 1

FROM Activity a1 JOIN Activity a2
ON a1.player_id=a2.player_id AND a1.event_date >= a2.event_date

这样得到的结果是a1表的时间是大于等于a2表的

如2016-03-01>=2016-03-01，结果是1个2016-05-02 >= 2016-05-02和2016-03-01，结果是两个，以此类推。这样正好实现了累加。

SELECT
    a1.player_id,
    a1.event_date,
    SUM(a2.games_played) AS games_played_so_far
FROM Activity AS a1 JOIN Activity AS a2
ON a1.player_id=a2.player_id AND a1.event_date>=a2.event_date
GROUP BY a1.player_id, a1.event_date;