UPDATE tblone t1, tblone t2
SET
t1.Column=(@temp:=t1.Column),
t1.Column = t2.Column ,
t2.Column = @temp
WHERE
t1.ID=10 and t2.ID=20

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

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

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

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

Works for all scenarios based on my quick tests.

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

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;

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."

"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.

In my particular case, I only wanted to modify a very small subset of the data.

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

vs

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

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