Sql自动增加数据库字段

Question

我需要一些帮助来更改一个大数据库的数据库字段中的数字。数据库结构是：

CREATE TABLE IF NOT EXISTS `some_product_category` (
  `product_category_id` int(255) unsigned NOT NULL AUTO_INCREMENT,
  `category_id` int(10) unsigned NOT NULL,
  `product_id` int(10) unsigned NOT NULL,
  `ordering` int(10) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`product_category_id`),
  UNIQUE KEY `category_id` (`category_id`,`product_id`),
  KEY `product_id` (`product_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1279345 ;

INSERT INTO `some_product_category` (`product_category_id`, `category_id`, `product_id`, `ordering`)
VALUES
(2, 1108, 2, 2),
(1, 1108, 1, 1),
(1279344, 1012, 1271372, 1),
(1271373, 1012, 1271373, 1),
(1271374, 1012, 1271374, 1),
(1271375, 1012, 1271375, 1),
(1271376, 1012, 1271376, 1),
(1271377, 1012, 1271377, 1),
(1271378, 1012, 1271378, 1),
(1271379, 1012, 1271379, 1),
(1271380, 1012, 1271380, 1),
(1271370, 1012, 1271370, 1),    

当category_id相同时，我需要更改排序字段。对于category_id 1012，一个必须是1，订购两个必须到2，订购三个必须是3等。