CREATE TABLE `articles` (
  `article_id` SMALLINT(6) UNSIGNED NOT NULL AUTO_INCREMENT,
  `category_id` SMALLINT(6) UNSIGNED NOT NULL DEFAULT '0',
  `title` VARCHAR(200),
  PRIMARY KEY (`article_id`)
)ENGINE=MyISAM
AUTO_INCREMENT=1 CHARACTER SET 'utf8' COLLATE 'utf8_general_ci';

CREATE TABLE `tags` (
  `tag_id` MEDIUMINT(9) UNSIGNED NOT NULL AUTO_INCREMENT,
  `title` VARCHAR(50) COLLATE utf8_general_ci NOT NULL DEFAULT '',
  PRIMARY KEY (`tag_id`),
  UNIQUE KEY `title` (`title`)
)ENGINE=MyISAM
AUTO_INCREMENT=1 CHARACTER SET 'utf8' COLLATE 'utf8_general_ci';

CREATE TABLE `tag_relations` (
  `article_id` SMALLINT(6) UNSIGNED NOT NULL DEFAULT '0',
  `tag_id` MEDIUMINT(9) UNSIGNED NOT NULL DEFAULT '0',
  PRIMARY KEY (`article_id`, `tag_id`)
)ENGINE=InnoDB
ROW_FORMAT=FIXED CHARACTER SET 'utf8' COLLATE 'utf8_general_ci';

SELECT a2.articles_id
FROM `tag_relations` AS a1
JOIN `tag_relations` AS a2
USING (tag_id)
WHERE a1.article_id = ?
AND a1.article_id <> a2.article_id
GROUP BY a2.article_id

SELECT a.article_id,
       a.title
FROM articles a
     JOIN 
     (SELECT tr.article_id, Count(*) as MatchCount FROM `tag_relations` tr
      WHERE tr.tag_id in (SELECT tag_id FROM tag_relations tr2 WHERE
       tr2.article_id = 1) GROUP BY tr.article_id) as sub ON a.article_id =
        sub.article_id
WHERE 
      a.`category_id` = '13' AND
      a.article_id <> '1'
ORDER BY sub.MatchCount desc