SQL查询帮助-多个联接

Question

对该SQL查询有麻烦。 情况如下：

我有三个表，其结构如下：

Events -> fields ID and Name, 
Players -> fields ID and Name, 
Matches -> fields ID, EventID, Player1ID, Player2ID

我想执行一个查询，向我显示匹配表，但将EventID替换为Event.Name，将Player1ID替换为Players.Name，将Player2ID替换为Players.Name。

我很容易使用以下方法获得一名球员和赛事：

SELECT 
   Players.Name as Player1, Events.Name 
FROM 
   (Matches 
INNER JOIN 
   Events ON (Matches.EventID=Events.ID))  
INNER JOIN 
   Players ON (Matches.Player1ID = Player.ID) ;

但是，如何获得Player2的名称？

Answer 1

将第二个JOIN添加到Players表中：

SELECT 
   Players.Name as Player1, Events.Name, 
   p2.Name as Player2
FROM 
   Matches 
INNER JOIN 
   Events ON Matches.EventID = Events.ID
INNER JOIN 
   Players ON Matches.Player1ID = Player.ID
INNER JOIN 
   Players p2 ON Matches.Player2ID = p2.ID;

Answer 2

您可以通过将表连接在一起来做到这一点。 诀窍是您必须两次包含Players表。 在这种情况下，您需要使用表别名来区分from子句中对表的这两个引用：

select m.matchid, e.name as event_name, p1.name as player1_name, p2.name as player2_name
from matches m join
     events e
     on m.eventid = e.id join
     players p1
     on m.player1 = p1.id join
     players p2
     on m.player2 = p2.id;

我还为其他表添加了表别名，这使查询更易于阅读。

Answer 3

SELECT p1.Name, p2.Name, Events.Name
FROM Matches
    INNER JOIN Events ON (Matches.EventID=Events.ID))  
    INNER JOIN Players p1 ON (Matches.Player1ID = p1.ID)
    INNER JOIN Players p2 ON (Matches.Player2ID = p2.ID)

Answer 4

您需要使用Player2ID字段上的Players表再次加入查询。 因此，将查询更改为：

SELECT one.Name as Player1, two.Name as Player2, Events.Name 
FROM  
Matches INNER JOIN 
Events ON Matches.EventID=Events.ID INNER JOIN 
Players one ON Matches.Player1ID = one.ID INNER JOIN
Players two ON Matches.Player1ID = two.ID