使用多个联接而不使用子查询的SQL查询

假设我有三个包含这些列的表

      Players - id, name
      Events - id, name
      Games - first_player_id, second_player_id, event_id. 

我需要

玩家

在

事件

中进行的

游戏

的详细信息。 我可以写这样的查询

SELECT players.id, events.id as event_id, 
(SELECT name as player_one_name from players where id = games.first_player_id), 
(SELECT name as player_two_name from players where id = games.second_player_id), 
games.id as game_id  
    FROM events 
    INNER JOIN games on events.id = games.event_id 
    INNER JOIN players on games.first_player_id = players.id;"

这里我使用两个子查询来获取玩家的名字并给出正确的结果。这个查询可以优化吗？例如，我可以删除任何

子查询

或

内部连接

仅供参考，我使用

PostgreSQL

数据库

---

谢谢。

如果您不想在select语句中使用子查询，则必须为每个子集提供联接。因为您的数据库是面向集合的，所以两个内部联接将被证明更有效

SELECT players.id, events.id as event_id,
     player_one_name=player_one.name,
     player_tow_name=player_two.name
FROM events 
INNER JOIN games on events.id = games.event_id 
INNER JOIN players player_one on games.first_player_id = player_one.id
INNER JOIN players player_two on games.second_player_id = player_two.id

---

必须为每个外键执行联接

SELECT players_a.id, events.id as event_id, 
players_a.name as player_one_name,
players_b.name as player_two_name,
games.id as game_id  
    FROM events 
    INNER JOIN games on events.id = games.event_id 
    INNER JOIN players players_a on games.first_player_id = players.id
    INNER JOIN players players_b on games.first_player_id = players.id

---

目前公认的答案是关于两次加入

players

表是正确的，但在其他方面大多是错误的。这将有助于：

SELECT e.id AS event_id
      ,g.id AS game.id
      ,p1.name AS first_player
      ,p2.name AS second_player
FROM   events e
LEFT   JOIN games    g ON g.event_id = e.id 
LEFT   JOIN players p1 ON p1.id = g.first_player_id 
LEFT   JOIN players p2 ON p2.id = g.second_player_id;

• 使用

  LEFT[OUTER]JOIN

  覆盖事件没有游戏或游戏没有两个玩家（尚未）的情况
• 使用表别名简化语法。要两次加入同一个表，还需要至少一个表别名。
  将别名附加到

  FROM

  列表中的表后，在查询中只有该别名可见，而不是表的原始名称

---

非常感谢！将在30秒内接受您的回答：P:DMy的目标是让参加某个活动的玩家参加游戏。因此，如果事件中有玩家，并且游戏中有该事件，我只需要行。如果我加入左边，我也会得到没有玩家的游戏和事件，对吗？@BharathMg:没错。然后把

左键放下。