SQL 计数两列并相减

Question

晚安。我遇到了这个问题，我尝试了子查询和案例但没有成功。

当玩家在我的游戏中杀人时，数据库中会添加一条新记录，这条记录包含杀手的 ID 和被杀死的玩家的 ID，所以过一会儿 table 会变成这样这个：

killer_id  |  death_id
100        |  200        -> (Player 100 Killed player 200)
100        |  200
100        |  200
200        |  100
300        |  100
300        |  200

我试图根据他们的击杀数减去他们的死亡数来计算每个玩家的分数，因此根据以上数据：

Rank:
Pos 1. player 300 / 2 Points  (2 Kills - 0 deaths)
Pos 2. player 100 / 1 Point   (3 Kills - 2 deaths)
Pos 3. player 200 / -3 Points (1 Kills - 4 deaths)

我想在一个 SQL 查询上执行此操作，但我被卡住了...任何提示都将不胜感激。

Answer 1

您可以使用 union all 和聚合：

select player_id, sum(cnt) score
from (
    select killer_id player_id, 1 cnt from mytable
    union all select death_id, -1 from mytable
) t
group by player_id
order by score desc

如果你还想要玩家排名，你可以使用window函数（这需要MySQL 8.0）：

select rank() over(order by sum(cnt) desc) pos, player_id, sum(cnt) score
from (
    select killer_id player_id, 1 cnt from mytable
    union all select death_id, -1 from mytable
) t
group by player_id
order by score desc

Answer 2

您可以使用 union all 和聚合：

select player_id, sum(points)
from ((select killer_id as player_id, 1 as points
       from t
      ) union all
      (select death_id, -1 as points
       from t
      ) 
     ) kd
group by player_id
order by sum(points) desc;

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