MySQL query: Updating foreign key with unique Id - mysql

Question

I need to link two tables 1-to-1, but the values that are to be compared and linked upon, are not unique.
I cannot find a way. As an example, I added a very simple version.
CREATE TABLE `T1` (
`id` int(6) unsigned NOT NULL,
`cmp` int(3) NOT NULL,
`uniqueT2Id` int(3) unsigned,
PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
CREATE TABLE IF NOT EXISTS `T2` (
`id` int(6) unsigned NOT NULL,
`cmp` int(3) NOT NULL,
PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `T1` (`id`, `cmp`, `uniqueT2Id`) VALUES
('1', '1', NULL),
('2', '1', NULL),
('3', '2', NULL),
('4', '3', NULL),
('5', '1', NULL);
INSERT INTO `T2` (`id`, `cmp`) VALUES
('1', '1'),
('2', '1'),
('3', '1'),
('4', '2'),
('5', '3');
UPDATE T1 SET uniqueT2Id=
(SELECT id FROM T2 WHERE T2.cmp=T1.cmp AND
id NOT IN (SELECT * FROM
(SELECT uniqueT2Id FROM T1 WHERE uniqueT2Id IS NOT NULL) X)
ORDER BY id ASC LIMIT 1);
SELECT * FROM T1;
http://sqlfiddle.com/#!9/3bab7c/2/0
The result is
id cmp uniqueT2Id
1 1 1
2 1 1
3 2 4
4 3 5
5 1 1
I want it to be
id rev uniqueT2Id
1 1 1
2 1 2
3 2 4
4 3 5
5 1 3
In the UPDATE I try to pick an Id that is not already used, but this obviously does not work. Does anyone know a way to do this in MySQL, preferrably without PHP?

Answer 1

I found an answer myself, with variables. It is horrible and requires a dummy field in the table, but it works. I am open for improvements.
CREATE TABLE `T1` (
`id` int(6) unsigned NOT NULL,
`cmp` int(3) NOT NULL,
`uniqueT2Id` int(3) NULL,
`dummy` varchar(200) NULL,
PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
CREATE TABLE IF NOT EXISTS `T2` (
`id` int(6) unsigned NOT NULL,
`cmp` int(3) NOT NULL,
PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `T1` (`id`, `cmp`, `uniqueT2Id`) VALUES
('1', '1', NULL),
('2', '1', NULL),
('3', '2', NULL),
('4', '5', NULL),
('5', '3', NULL),
('6', '1', NULL);
INSERT INTO `T2` (`id`, `cmp`) VALUES
('1', '1'),
('2', '1'),
('3', '1'),
('4', '2'),
('5', '3');
SET #taken = '/' ;
UPDATE T1
SET uniqueT2Id= #new:=
(SELECT id FROM T2 WHERE T2.cmp=T1.cmp AND
INSTR(#taken, CONCAT('/',id,'/')) = 0
ORDER BY id ASC LIMIT 1),
dummy=IF(#new IS NOT NULL,#taken:=CONCAT(#taken, #new, "/"),NULL);
http://sqlfiddle.com/#!9/4a61d/1/0