How to Swap Values in MySQL without Unique Constraint Violations?

Swapping Values in MySQL without Unique Constraint Violations

In MySQL, swapping values between two rows can be challenging when a unique constraint is applied to the affected column. The issue arises because MySQL enforces constraint checks after each row update, rather than the completion of the entire statement.

To address this limitation, consider the following solution:

<code class="sql">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 ;</code>

In this solution, a transaction is initiated to ensure that both updates are performed atomically. Negative values are temporarily assigned to the priorities to avoid conflicts with the unique constraint. Subsequently, another update assigns the negative values to the original priorities, effectively swapping their values.

Alternatively, if negative values are not permitted in the priority column, you can use temporary variables or create a temporary table to hold the swapped values. However, these methods may involve multiple queries.

It's important to note that removing the unique constraint temporarily is not recommended, as it can compromise data integrity.

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