SQL查詢幫助-多個聯接

Question

對該SQL查詢有麻煩。 情況如下：

我有三個表，其結構如下：

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

我想執行一個查詢，向我顯示匹配表，但將EventID替換為Event.Name，將Player1ID替換為Players.Name，將Player2ID替換為Players.Name。

我很容易使用以下方法獲得一名球員和賽事：

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，如何獲得Player2的名稱？

Answer 1

將第二個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

您可以通過將表連接在一起來做到這一點。 訣竅是您必須兩次包含Players表。 在這種情況下，您需要使用表別名來區分from子句中對表的這兩個引用：

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;

我還為其他表添加了表別名，這使查詢更易於閱讀。

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表再次加入查詢。 因此，將查詢更改為：

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