有条件地选择MYSQL列 [英] Selecting MYSQL column conditionally

问题描述

我的问题的示例架构在 SQLFiddle 中给出，如下所示:

Example schema for my question is given at SQLFiddle and as follow:

CREATE TABLE `players` (
  `player1id` int(11) NOT NULL,
  `player2id` int(11) NOT NULL,
  PRIMARY KEY (`player1id`,`player2id`)
) ENGINE=InnoDB;

INSERT INTO `players` values
(1,5),
(1,7),
(5,3),
(5,4),
(2,1);

在在线游戏中，我想选择对手，该对手可以在玩家player1id或player2id中.

In a online game, I want to select opponent, which could be in player player1id or player2id.

需要示例输入/输出

Input 1: Output 5, 7, 2
Input 5: Output 1, 3 ,4
& so on.

也就是说，所需的数据可以在任何列中，但是我需要有条件地或以任何其他方式在单列中输出.我听说过MySQL条件列，但无法创建查询以获取所需的输出.有人可以帮忙进行必要的查询吗?

That is, required data could be in any column but I need output in single column conditionally or any other way. I heard about MySQL conditional column but could not create query to get required output. Can someone please help with required query.

修改

基于此链接，我运行以下命令查询，但失败.

Based on this link, I run following query but failed.

SELECT IF(PLAYER1ID IS 1,PLAYER2ID as opponent,PLAYER1ID as opponent) 
FROM players
WHERE PLAYER1ID = 1 OR PLAYER2ID = 1;

推荐答案

我认为您可以使用case when then语法，如下所示:

I think you may use case when then syntax as below:

SELECT CASE WHEN player1id = 1 THEN player2id ELSE player1id  END
FROM players WHERE player1id =1 OR player2id=1;

或者:

SELECT CASE WHEN player1id = 1 THEN player2id WHEN player2id =1 THEN player1id  END
FROM players WHERE player1id =1 OR player2id=1;

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