Question

I have a “tasks” table with a priority column, which has a unique constraint.

I’m trying to swap the priority value of two rows, but I keep violating the constraint. I saw this statement somewhere in a similar situation, but it wasn’t with MySQL.

UPDATE tasks 
SET priority = 
CASE
    WHEN priority=2 THEN 3 
    WHEN priority=3 THEN 2 
END 

WHERE priority IN (2,3);

This will lead to the error:

Error Code: 1062. Duplicate entry '3' for key 'priority_UNIQUE'

Is it possible to accomplish this in MySQL without using bogus values and multiple queries?

EDIT:

Here’s the table structure:

CREATE TABLE `tasks` (
  `id` int(11) NOT NULL,
  `name` varchar(200) DEFAULT NULL,
  `priority` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `priority_UNIQUE` (`priority`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

Answer 1

Is it possible to accomplish this in MySQL without using bogus values and multiple queries?

No. (none that I can think of).

The problem is how MySQL processes updates. MySQL (in difference with other DBMS that implement UPDATE properly), processes updates in a broken manner. It enforces checking of UNIQUE (and other) constraints after every single row update and not – as it should be doing – after the whole UPDATE statement completes. That’s why you don’t have this issue with (most) other DBMS.

For some updates (like increasing all or some ids, id=id+1), this can be solved by using – another non-standard feature – an ORDER BY in the update.

For swapping the values from two rows, that trick can’t help. You’ll have to use NULL or a bogus value (that doesn’t exist but is allowed in your column) and 2 or 3 statements.

You could also temporarily remove the unique constraint but I don’t think that’s a good idea really.

---

So, if the unique column is a signed integer and there are no negative values, you can use 2 statements wrapped up in a transaction:

START TRANSACTION ;
    UPDATE tasks 
    SET priority = 
      CASE
        WHEN priority = 2 THEN -3 
        WHEN priority = 3 THEN -2 
      END 
    WHERE priority IN (2,3) ;

    UPDATE tasks 
    SET priority = - priority
    WHERE priority IN (-2,-3) ;
COMMIT ;