如何使用MySQL if语句作为列名？

Question

I have table:

mysql> DESCRIBE swaps;
+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| id          | int(11)  | NO   | PRI | NULL    | auto_increment |
| user1_id    | int(11)  | NO   |     | NULL    |                |
| user2_id    | int(11)  | NO   |     | NULL    |                |
| hasto       | int(11)  | NO   |     | NULL    |                |
| requested   | datetime | NO   |     | NULL    |                |
| accepted    | datetime | YES  |     | NULL    |                |
| swapped1    | datetime | YES  |     | NULL    |                |
| swapped2    | datetime | YES  |     | NULL    |                |
| rejected    | datetime | YES  |     | NULL    |                |
| rejected_by | int(11)  | YES  |     | NULL    |                |
+-------------+----------+------+-----+---------+----------------+

where user1_id and user2_id represents users that are swapping and swapped1/swapped2 represent time when they affirm swap. I need to create mechanism to close swap when both have confirmed swap.

I need something like this:

UPDATE swaps SET if(user1_id = ?, swapped1, swapped2) = ? WHERE id=?

where ? marks represent current user, current time, swap_id. What I want is to insert time into swapped1 if current user is user1 and into swapped2 if current user is user2.

Running this query I get error:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'if(user1_id = '3', swapped1, swapped2) = '2017-01-13 07:34:08' WHERE id='3'' at line 1

what means that I am not using if statement correctly. Is there a way to perform if statement on column name?

Answer 1

You can't use IF on the left side of an assignment, but you can update both columns conditionally:

UPDATE swaps
   SET swapped1 = IF(user1_id = [current_user], [current_time], swapped1)
     , swapped2 = IF(user2_id = [current_user], [current_time], swapped2)
 WHERE id = [swap_id];

Answer 2

You must test always user1_id like this:

UPDATE swaps 
    SET 
        swapped1 if(user1_id = 3, '2017-01-13 07:34:08' , swapped1),
        swapped2 if(user1_id = 3, swapped2, '2017-01-13 07:34:08' )

WHERE id=3;