Selecting MYSQL column conditionally

Question

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,
         


        

Answer 1

This ought to give you what you're looking for:

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;

Answer 2

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;

Alternatively:

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

Answer 3

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

SQL FIDDLE DEMO