Remaping numbers in table all at once - mysql

Question

I was converting a database structure to a different one. There's this problem with ID's. The items used to reference to each other with ID - but these ID's must change because the new structure is different.
So you have a table (this is an example):
| id | ref |
+----+-----+
| 1 | 3 |
| 2 | 1 |
| 3 | 1 |
| 4 | 2 |
Now let's say that in new table, the refference id's must change like this:
| old | new |
+-----+-----+
| 1 | 2 |
| 2 | 1 |
| 3 | 4 |
| 4 | 3 |
See what happens if you subsequently UPDATE to replace old ref with new ones:
Replacing: 1=>2 Replacing: 2=>1 This is what we need at the end:
well, damn
| id | ref | | id | ref | | id | ref |
+----+-----+ +----+-----+ +----+-----+
| 1 | 3 | | 1 | 3 | | 1 | 4 |
| 2 | *2* | | 2 | 2 | | 2 | 2 |
| 3 | *2* | | 3 | 2 | | 2 | 2 |
| 4 | 2 | | 4 | *2* | | 2 | 1 |
So I need to replace it at one time. How can I do this?

Answer 1

Since the criteria don't matter, this will do the trick. The core of the solution is: update all rows in one go.
update YourTable
set ref =
case ref
when 1 then 2
when 2 then 1
when 3 then 4
when 4 then 3
end
If that is not possible:
add a new column 'newref'
update 'newref' in any way you want to contain the new ids
update the 'ref' column with the values from 'newref'
drop the 'newref'

Answer 2

This update all in one instruction
UPDATE t1 a
INNER JOIN t2 b
ON a.ref=b.old
SET a.ref=b.new;
Result
select * from t1
id ref
1 4
2 2
3 2
4 1
http://sqlfiddle.com/#!2/ab49c/1
Schema and data sample
create table t1 (id int, ref int);
create table t2 (old int, new int);
insert t1 values(1,3);
insert t1 values(2,1);
insert t1 values(3,1);
insert t1 values(4,2);
insert t2 values(1,2);
insert t2 values(2,1);
insert t2 values(3,4);
insert t2 values(4,3);