思路
pid和eventday不变，找到pid相同且eventday少的即可求sum
sql
# Write your MySQL query statement below select player_id, event_date, (select sum(games_played) from Activity as b where b.player_id = a.player_id and b.event_date <= a.event_date) as games_played_so_far from Activity as a
注意：力扣的顺序不讲究，测试不一定通过，提交通过即可
但是这个答案很慢，想想怎么优化。。。
用另一种双表的思路解决
# Write your MySQL query statement below
select a.player_id, a.event_date, sum(b.games_played) as games_played_so_far
from Activity a, Activity b
where a.player_id = b.player_id
and a.event_date >= b.event_date
group by a.player_id, a.event_date #这是主键
总结：
好久没写sql了，第一次居然胡乱写对了
啊哈，冲冲冲
