Comments on: Swapping Column Values in MySQL

By: Alex

Alex — Wed, 13 Jan 2010 14:10:44 +0000

What about swapping the id column of table 1, with another column from table 2?

Can I apply any of these methods to swap columns across tables?

By: Maresa

Maresa — Tue, 13 Oct 2009 17:36:40 +0000

@Dogo: Swapping values between 2 rows from the same table is explained on my post here: http://www.microshell.com/database/sql/swap-values-in-2-rows-sql/

In a sense, it's basically the same solution as this post. You do self join on the table, then swap columns.

By: Artem Russakovskii

Artem Russakovskii — Sat, 01 Aug 2009 03:40:58 +0000

@Dogo
Try posting this at http://www.stackoverflow.com. Should have a discussion going in no titme.

By: Dogo

Dogo — Sat, 01 Aug 2009 01:25:29 +0000

Hi,

i looked around a lot but do not seem to get it right:
I'd like to swap two values in different lines. As entry id 14 (e.g) has "order" 5 and another has "order" 6.
Is there any way to swap these two within one single query? I already have the id (14) and order of the first one.

I ran through different approaches like "ORDER BY order DESC LIMIT 1" but it always takes two queries…

Best Regards,

Dogo

By: Handy

Handy — Thu, 14 May 2009 21:42:52 +0000

I've used
('1', 'a', '10')
, ('2', null, '20')
, ('3', 'c', '30')
It's great !

By: Mathijs

Mathijs — Wed, 29 Apr 2009 13:42:30 +0000

maybe you can do something with this aswell
had some problems with switching integer values in 2 different rows with eachother.
they are used to describe an order, and eacht time there will be a difference of one.
this is what did the trick.

UPDATE tablename AS first, tablename AS second, tablename AS third
SET first.ord = second.ord, second.ord = third.ord
WHERE first.parent = second.parent AND first.id = 1 AND first.id = third.id AND second.ord = third.ord-1 AND first.status > 0 AND second.status > 0

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Thu, 19 Mar 2009 02:00:35 +0000

[...] Swapping Column Values in MySQL [...]

By: custom software developer

custom software developer — Mon, 02 Mar 2009 11:49:29 +0000

Thanks for that helpful post. that was awesome I shall necessarily try

By: Peter Gulutzan

Peter Gulutzan — Sun, 22 Feb 2009 00:58:24 +0000

We're aware of the problem.
http://forge.mysql.com/worklog/task.php?id=927

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Log Buffer #136: A Carnival of the Vanities for DBAs — Fri, 20 Feb 2009 17:01:43 +0000

[...] On beer planet, Artem Russakovskii publishes his findings on swapping column values in MySQL. [...]

By: Intercambiando el valor de columnas en MYSQL | dominios, diseño web, ecommerce - Mantis Technology Solutions Blog

Thu, 19 Feb 2009 05:02:47 +0000

[...] Swapping Column Values in MySQL [...]

By: Artem Russakovskii

Artem Russakovskii — Wed, 18 Feb 2009 20:06:14 +0000

@Dipin
Brilliant. That indeed works for all test cases. I think this is finally the cleanest and most elegant solution.

By: Lenguajes X » Intercambiando el valor de columnas en MYSQL

Lenguajes X » Intercambiando el valor de columnas en MYSQL — Wed, 18 Feb 2009 19:28:40 +0000

[...] Swapping Column Values in MySQL [...]

By: Dipin Hora

Dipin Hora — Wed, 18 Feb 2009 18:10:14 +0000

update swap_test_1 set x=(@temp:=x), x = y, y = @temp;

Works for all scenarios based on my quick tests.

By: Artem Russakovskii

Artem Russakovskii — Wed, 18 Feb 2009 15:41:36 +0000

@Dmitri
You're absolutely right. I had it in the back of my mind to test the NULL case but it escaped me till now. So, approach #2 doesn't swap the values if one of them is NULL. Approach #3 still works fine. I'll make a note of it, until a new approach is developed for #2.

By: Dmitri Mikhailov

Dmitri Mikhailov — Wed, 18 Feb 2009 15:32:19 +0000

*** use this record set:

insert into swap_test_1 values
('1', 'a', '10')
, ('2', null, '20')
, ('3', 'c', '30')
;

By: Dmitri Mikhailov

Dmitri Mikhailov — Wed, 18 Feb 2009 15:29:10 +0000

I would not recommend to use this approach, as the update fails to swap values if one of them is null:

drop table if exists swap_test_1
;
create table swap_test_1
(
id int not null auto_increment
, x varchar(255) null
, y varchar(255) null
, primary key (id)
) engine=innodb;

insert into swap_test_1 values
('1', 'a', '10')
, ('2', 'b', null)
, ('3', 'c', '30')
;

update swap_test_1 set x=y, y=@temp where (@temp:=x) is not null;

select * from swap_test_1;

By: Pabloj

Pabloj — Wed, 18 Feb 2009 13:02:47 +0000

All major databases "think is sets" and make the swap seamless, if I remember well just MySQL and Firebird don't work this way.

By: Artem Russakovskii

Artem Russakovskii — Wed, 18 Feb 2009 10:04:00 +0000

@Roland
Yeah, that makes sense, and I thought the caveat was worth pointing out, seeing how some examples on the web are omitting parentheses.

As far as the first query, I just tested both MyISAM and InnoDB and they both behave the same – x and y end up equal, as expected. The MySQL manual confirms it: "Single-table UPDATE assignments are generally evaluated from left to right."

By: Roland Bouman

Roland Bouman — Wed, 18 Feb 2009 09:50:16 +0000

Hi!

"In the first query, the missing parentheses will cause data corruption. I will make sure to point it out and correct my example."

I think it's a matter of operator precedence. Look here: http://dev.mysql.com/doc/refman/5.1/en/operator-precedence.html

As you can see, the := binds weakest so omitting the parenthesis will cause

x IS NOT NULL

to be evaluated first. That expression evaluates to a boolean value, but MySQL doesn't have those so it becomes either 1 (TRUE) or 0 (FALSE)

As for the obvious first query,

UPDATE swap_test SET x=y, y=x;

do you notice any difference between InnoDB and MyISAM? Personally I think this query should just work but I am aware of the fact that MySQL takes a few shortcuts in the UPDATE implementation.