选择MYSQL列有条件

Question

我的问题

示例模式在SQLFiddle和如下给出：

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); 

在一个网络游戏，我要选择对手，这可能是球员player1id或player2id。

实施例输入/输出需要

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

也就是说，所需的数据可以是在任何列中，但我需要在有条件地单塔或任何其他方式输出。我听说过MySQL条件列，但无法创建查询来获取所需的输出。有人可以请求帮助所需的查询。

编辑

基于this link，我运行下面的查询，但失败了。

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

Answer 1

我想你可以使用如下case when then语法：

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; 

Answer 2

select player1id from players where player2id=1 
union 
select player2id from players where player1id=1 

SQL FIDDLE DEMO

Answer 3

这应该给你你在做什么looki ng用于：

SET @PlayerId = 1; 
SELECT `player2id` AS 'Other Player ID' 
    FROM `players` 
WHERE `player1id` = @PlayerId 
    AND `player2id` != @PlayerId 

UNION 
SELECT `player1id` 
    FROM `players` 
WHERE `player2id` = @PlayerId 
    AND `player1id` != @PlayerId; 


SET @PlayerId = 5; 
SELECT `player2id` AS 'Other Player ID' 
    FROM `players` 
WHERE `player1id` = @PlayerId 
    AND `player2id` != @PlayerId 

UNION 
SELECT `player1id` 
    FROM `players` 
WHERE `player2id` = @PlayerId 
    AND `player1id` != @PlayerId;