DROP TABLE IF EXISTS `scores`;

CREATE TABLE `scores` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `value` char(1) DEFAULT NULL,
  `level` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

LOCK TABLES `scores` WRITE;
INSERT INTO `scores` (`id`,`value`,`level`)
VALUES
	(1,'B',NULL),
	(2,'A',NULL),
	(3,'A',NULL),
	(4,'C',NULL),
	(5,'A',NULL);
UNLOCK TABLES;

/* Update level value of all A scores */
SET @rownum=0;
UPDATE scores t, (SELECT @rownum:=@rownum+1 rownum, scores.* FROM scores WHERE value='A') r
SET t.level = r.rownum WHERE (t.id = r.id);

/*
NOW scores is:

id     value    level
----------------------
1	B	NULL
2	A	1
3	A	2
4	C	NULL
5	A	3

*/