Specific merge 2 columns in sql excluding one of them

Question

I have table like this:

| user1 | user2 |
|   1   |   2   |
|   3   |   1   |
|   4   |   1   |
|   1   |   5   |
|  ...  |  ...  |

I wanna mer

Answer 1

SELECT user1 AS users FROM `table` WHERE user1 NOT IN (SELECT user2 FROM `table`)
 UNION 
SELECT user2 AS users FROM `table` WHERE user2 NOT IN (SELECT user1 FROM `table`)
ORDER BY users

Answer 2

SELECT IF(user1 = 1, user2, user1) AS user
FROM table
WHERE user1 = 1 OR user2 = 1