How to Swap Row Priority Values in MySQL with a Unique Constraint?

Swapping Values of Rows with Unique Constraints in MySQL

Question:

How can you swap the priority values of two rows in MySQL without violating the table's unique constraint on the priority column?

Background:

The tasks table has a priority column with a unique constraint, meaning each row can have a unique value in that column. When updating rows, MySQL checks for unique constraint violations after every update, which prevents swapping values directly.

Solution:

Unfortunately, it is not possible to swap values of rows directly in MySQL without violating the unique constraint or using bogus values and multiple queries. This is due to MySQL's unusual way of processing updates, which enforces uniqueness checks after each row update instead of after the update statement completes.

While it may be possible to solve similar issues with other DBMS using tricks like ORDER BY, this technique does not work for swapping values.

Alternatives:

Two alternative approaches can be considered:

1. NULL or Bogus Values: Use a NULL value or a bogus value that does not exist in the column. This requires using multiple queries to complete the swap.
2. Temporary Constraint Removal: Remove the unique constraint temporarily, perform the swap, and re-add the constraint. This is not a recommended approach as it can lead to data integrity issues.

Example (Signed Integer and No Negative Values):

If the priority column is a signed integer that does not contain negative values, you can use the following query wrapped in a transaction:

<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>

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