MySQL 计算多个分组列上的行数

Question

我的 MySQL 表有以下结构:

-------------------------------------------
                 Goal
-------------------------------------------
GoalID  |  GameID  |  ScorerID  |  AssistID
-------------------------------------------
   1          1          1            2
   2          1          2            3
   3          1          2           null
   4          1          3            2
   5          2          1            3
   6          2          1           null


----------------------
       Player
----------------------
PlayerID  |  LastName
----------------------
   1            AB    
   2            CD  
   3            EF  

现在，我想要实现的是:

-------------------------------------------
GameID  |  Player  |  Goals  |  Assists
-------------------------------------------
   1         AB          1         0
   1         CD          2         2
   1         EF          1         1
   2         AB          2         0
   2         EF          0         1

这是我尝试过的方法，但它给了我错误的结果:

SELECT Goal.GoalID,
   Goal.GameID,
   COUNT(Scorer.PlayerID) AS Goals,
   COUNT(Assist.PlayerID) AS Assists,
   Player.LastName AS Player
FROM Goal
LEFT JOIN Player Player
       ON Player.PlayerID = Goal.ScorerID
       OR Player.PlayerID = Goal.AssistID
LEFT JOIN Player Scorer
       ON Scorer.PlayerID = Goal.ScorerID
LEFT JOIN Player Assist
       ON Assist.PlayerID = Goal.AssistID
GROUP BY Player,
         Goal.GameID
ORDER BY Goal.GameID,
         Goal.GoalID

我很确定有一种简单的方法可以得到结果，但我就是无法理解这个方法。

Answer 1

您可以使用条件聚合来完成此操作。为此，您在球员id与scorerid或assist id匹配的情况下加入球员和目标表，然后获得球员id在score或assist列中的情况的SUM()，最终分组游戏 ID 和玩家 ID。像这样:

SELECT g.gameID, p.lastName AS Player, SUM(g.scorerID = p.playerID) AS goals, SUM(g.assistID = p.playerID) AS assists
FROM goal g
JOIN player p ON p.playerID = g.scorerID OR p.playerID = g.assistID
GROUP BY g.gameID, p.playerID;

这是一个 SQL Fiddle示例。

请务必注意，此示例假设玩家至少有一个进球或至少有一次助攻。如果您想要两者都没有的玩家，您可以使用外部连接。