Question

The accepted answer to sql swap primary key values fails with the error Can't reopen table: 't' – presumably this has something to do with opening the same table for writing twice, causing a lock.

Is there any shortcut, or do I have to get both, set one of them to NULL, set the second one to the first one, then set the first one to the previously fetched value of the second?

Answer 1

To swap id values of 1 and 2, I would use a SQL statement like this:

EDIT : this does NOT work on an InnoDB table, only works on a MyISAM table, per my testing.

UPDATE mytable a 
  JOIN mytable b ON a.id = 1 AND b.id = 2 
  JOIN mytable c ON c.id = a.id
   SET a.id = 0
     , b.id = 1
     , c.id = 2 

For this statement to work, the id value of 0 must not exist in the table, any unused value would be suitable… but to get this to work in a single SQL statement, you need to (temporarily) use a third id value.

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This solution works for regular MyISAM tables, not temporary tables. I missed that this was being performed on a temporary table, I was confused by the error message you reported Can't reopen table:.

To swap id values 1 and 2 in a temporary table, I’d run three separate statements, again, using a temporary placeholder value of 0:

UPDATE mytable a SET a.id = 0 WHERE a.id = 1;
UPDATE mytable b SET b.id = 1 WHERE b.id = 2;
UPDATE mytable c SET c.id = 2 WHERE c.id = 0;

Edit: Fixed errors