Swapping column values in MySQL

I just had to deal with the same and I'll summarize my findings.

1. The UPDATE table SET X=Y, Y=X approach obviously doesn't work, as it'll just set both values to Y.

2. Here's a method that uses a temporary variable. Thanks to Antony from the comments of http://beerpla.net/2009/02/17/swapping-column-values-in-mysql/ for the "IS NOT NULL" tweak. Without it, the query works unpredictably. See the table schema at the end of the post. This method doesn't swap the values if one of them is NULL. Use method #3 that doesn't have this limitation.

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

3. This method was offered by Dipin in, yet again, the comments of http://beerpla.net/2009/02/17/swapping-column-values-in-mysql/. I think it’s the most elegant and clean solution. It works with both NULL and non-NULL values.

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

4. Another approach I came up with that seems to work:

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

Essentially, the 1st table is the one getting updated and the 2nd one is used to pull the old data from.
Note that this approach requires a primary key to be present.

This is my 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);

The following code works for all scenarios in my quick testing:

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

UPDATE table SET X=Y, Y=X will do precisely what you want (edit: in PostgreSQL, not MySQL, see below). The values are taken from the old row and assigned to a new copy of the same row, then the old row is replaced. You do not have to resort to using a temporary table, a temporary column, or other swap tricks.

@D4V360: I see. That is shocking and unexpected. I use PostgreSQL and my answer works correctly there (I tried it). See the PostgreSQL UPDATE docs (under Parameters, expression), where it mentions that expressions on the right hand side of SET clauses explicitly use the old values of columns. I see that the corresponding MySQL UPDATE docs contain the statement "Single-table UPDATE assignments are generally evaluated from left to right" which implies the behaviour you describe.

Good to know.

Ok, so just for fun, you could do this! (assuming you're swapping string values)

mysql> select * from swapper;
+------+------+
| foo  | bar  |
+------+------+
| 6    | 1    | 
| 5    | 2    | 
| 4    | 3    | 
+------+------+
3 rows in set (0.00 sec)

mysql> update swapper set 
    -> foo = concat(foo, "###", bar),
    -> bar = replace(foo, concat("###", bar), ""),
    -> foo = replace(foo, concat(bar, "###"), "");

Query OK, 3 rows affected (0.00 sec)
Rows matched: 3  Changed: 3  Warnings: 0

mysql> select * from swapper;
+------+------+
| foo  | bar  |
+------+------+
| 1    | 6    | 
| 2    | 5    | 
| 3    | 4    | 
+------+------+
3 rows in set (0.00 sec)

A nice bit of fun abusing the left-to-right evaluation process in MySQL.

Alternatively, just use XOR if they're numbers. You mentioned coordinates, so do you have lovely integer values, or complex strings?

Edit: The XOR stuff works like this by the way:

update swapper set foo = foo ^ bar, bar = foo ^ bar, foo = foo ^ bar;

Two alternatives 1. Use a temporary table 2. Investigate the XOR algorithm

ALTER TABLE table ADD COLUMN tmp;
UPDATE table SET tmp = X;
UPDATE table SET X = Y;
UPDATE table SET Y = tmp;
ALTER TABLE table DROP COLUMN tmp;

Edit: About Greg's comment: No, this doesn't work:

mysql> select * from test;
+------+------+
| x    | y    |
+------+------+
|    1 |    2 |
|    3 |    4 |
+------+------+
2 rows in set (0.00 sec)


mysql> update test set x=y, y=x;
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from test;
+------+------+
| x    | y    |
+------+------+
|    2 |    2 |
|    4 |    4 |
+------+------+
2 rows in set (0.00 sec)

I've not tried it but

UPDATE tbl SET @temp=X, X=Y, Y=@temp

Might do it.

Mark

This surely works! I've just needed it to swap Euro and SKK price columns. :)

UPDATE tbl SET X=Y, Y=@temp where @temp:=X;

The above will not work (ERROR 1064 (42000): You have an error in your SQL syntax)

In SQL Server, you can use this query:

update swaptable 
set col1 = t2.col2,
col2 = t2.col1
from swaptable t2
where id = t2.id

As other answers point out, a simple swap won't work with MySQL because it caches the value of column 1 immediately before processing column 2, resulting in both columns being set to the value of column 2.

Given that the order of operations is not guaranteed in MySQL, using a temporary variable is also not reliable.

The only safe way to swap two columns without modifying the table structure is with an inner join, which requires a primary key (id in this case).

UPDATE table1 t1, table2 t2
SET t1.column1 = t1.column2,
    t1.column2 = t2.column1
WHERE t1.id = t2.id;

This will work without any issues.

Assuming you have signed integers in your columns, you may need to use CAST(a ^ b AS SIGNED), since the result of the ^ operator is an unsigned 64-bit integer in MySQL.

In case it helps anyone, here's the method I used to swap the same column between two given rows:

SELECT BIT_XOR(foo) FROM table WHERE key = $1 OR key = $2

UPDATE table SET foo = CAST(foo ^ $3 AS SIGNED) WHERE key = $1 OR key = $2

where $1 and $2 are the keys of two rows and $3 is the result of the first query.

Table name is customer. fields are a and b, swap a value to b;.

UPDATE customer SET a=(@temp:=a), a = b, b = @temp

I checked this is working fine.

You can apply below query, It worked perfect for me.

Table name: studentname
only single column available: name


update studentnames 
set names = case names 
when "Tanu" then "dipan"
when "dipan" then "Tanu"
end;

or

update studentnames 
set names = case names 
when "Tanu" then "dipan"
else "Tanu"
end;

This example swaps start_date and end_date for records where the dates are the wrong way round (when performing ETL into a major rewrite, I found some start dates later than their end dates. Down, bad programmers!).

In situ, I'm using MEDIUMINTs for performance reasons (like Julian days, but having a 0 root of 1900-01-01), so I was OK doing a condition of WHERE mdu.start_date > mdu.end_date.

The PKs were on all 3 columns individually (for operational / indexing reasons).

UPDATE monitor_date mdu
INNER JOIN monitor_date mdc
    ON mdu.register_id = mdc.register_id
    AND mdu.start_date = mdc.start_date
    AND mdu.end_date = mdc.end_date
SET mdu.start_date = mdu.end_date, mdu.end_date = mdc.start_date
WHERE mdu.start_date > mdu.end_date;

Let's say you want to swap the value of first and last name in tb_user.

The safest would be:

1. Copy tb_user. So you will have 2 tables: tb_user and tb_user_copy
2. Use UPDATE INNER JOIN query

UPDATE tb_user a
INNER JOIN tb_user_copy b
ON a.id = b.id
SET a.first_name = b.last_name, a.last_name = b.first_name

if you want to swap all the columns where x is to y and y to x; use this query.

UPDATE table_name SET column_name = CASE column_name WHERE 'value of col is x' THEN 'swap it to y' ELSE 'swap it to x' END;

Let's imagine this table and let's try to swap the m and f from the 'sex' table:

UPDATE sex
SET sex = CASE sex
WHEN 'm' THEN 'f'
ELSE 'm'
END;

So the updated table becomes: