How to Efficiently Swap Column Values in MySQL Without Changing the Table Structure?

Swapping Column Values in MySQL Without Altering Table Structure

As you mentioned, using UPDATE to swap column values with a simple SET X=Y, Y=X will not yield the desired result. However, there are several alternative approaches you can consider, depending on your data and permission limitations.

Method 1: Temporary Variable with IS NOT NULL Check

This method employs a temporary variable to hold the value of one column while swapping it with the other. However, it only works when both values are non-NULL.

UPDATE swap_test SET x=y, y=@temp WHERE (@temp:=x) IS NOT NULL;

Method 2: Temporary Variable Without IS NOT NULL Check

A more versatile method that handles both NULL and non-NULL values:

UPDATE swap_test SET x=(@temp:=x), x = y, y = @temp;

Method 3: Dual Table Update

Another method involves using a second table to swap the values. This requires a primary key to be present:

UPDATE swap_test s1, swap_test s2 SET s1.x=s1.y, s1.y=s2.x WHERE s1.id=s2.id;

Test Example

Using the provided test schema:

CREATE TABLE `swap_test` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `x` varchar(255) DEFAULT NULL,
  `y` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB;

INSERT INTO `swap_test` VALUES ('1', 'a', '10');
INSERT INTO `swap_test` VALUES ('2', NULL, '20');
INSERT INTO `swap_test` VALUES ('3', 'c', NULL);

Applying Method 2 should successfully swap the values:

UPDATE swap_test SET x=(@temp:=x), x = y, y = @temp;

Output:

id  x  y
1  10  a
2  20  NULL
3  NULL  c

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