How to Swap Row Values in MySQL with a Unique Constraint Without Violating Integrity?

Swapping Row Values in MySQL While Preserving Unique Constraint

In MySQL, the task of swapping priority values between two rows in a table with a unique constraint can encounter errors due to constraint violations. To understand the issue, let's examine a typical UPDATE statement:

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

WHERE priority IN (2,3);

This statement, however, results in an error:

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

Nature of the Issue

MySQL processes updates differently from other DBMS. It checks for constraint violations after each row update, rather than after the entire statement completes. As a result, swapping values directly violates the unique constraint.

Alternatives without Bogus Values or Multiple Queries

Unfortunately, there is no way to perform this operation without using an intermediate value (bogus or null) or multiple queries in MySQL. The unique constraint enforces uniqueness after each row update, making direct swapping impossible.

Temporary Constraint Removal

One option is to temporarily remove the unique constraint, execute the swap operation, and then re-add the constraint. However, this approach is not recommended as it compromises data integrity.

Bogus Values and Multiple Queries

The recommended approach involves using a bogus value (-3) to temporarily hold the first row's priority value and swapping it with the second row's value. Two queries within a transaction are required:

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 ;

This process ensures that the constraint is not violated during the swap operation.

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