MySQL many to many relationship issue - mysql

Question

I'm having three tables: articles, tags and articles_tags. As you can imagine, each article can have multiple tags and each tag can be assigned to multiple articles. I have so-called main article (represented by unique URL) and would like to get related articles of it, based on shared tags between them like: if main article and article 2 has one tag in common, show both articles (and ideally, it would not show/include in the results the main article). Unique URL of main article is passed in SQL query.
The expected result is beyond my reach, so any help would be appreciated.
SQLFiddle
Copied code, if site above goes offline:
Databases and content:
CREATE TABLE `articles` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`url` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`title` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`status` tinyint(4) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
CREATE TABLE `tags` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`tag` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
`url` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
CREATE TABLE `articles_tags` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`article_id` int(11) NOT NULL,
`tag_id` int(11) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
INSERT INTO `articles` (`url`, `title`, `status`) VALUES
('test-article-1', 'Test Article #1', 1),
('test-article-2', 'Test Article #2', 1),
('test-article-3', 'Test Article #3', 0),
('test-article-4', 'Test Article #4', 0),
('test-article-5', 'Test Article #5', 1);
INSERT INTO `tags` (`tag`, `url`) VALUES
('Test', 'test'),
('City', 'city'),
('Nature', 'nature');
INSERT INTO `articles_tags` (`article_id`, `tag_id`) VALUES
(1, 1),
(1, 2),
(1, 3),
(2, 2),
(3, 1),
(3, 2),
(4, 2),
(5, 1);
Latest (not working properly) SQL query:
SELECT
tags.tag,
articles.url,
articles.title
FROM articles
LEFT JOIN articles_tags ON articles_tags.article_id=articles.id
LEFT JOIN tags ON articles_tags.tag_id=tags.id
WHERE (articles.url='test-article-1'
OR tags.id IN (articles_tags.tag_id))
AND articles.status=1
GROUP BY articles.id
Result:
As you can see on SQLFiddle, it shows articles 1, 2 and 5, but in my mind it should show only 1 and 5
Expected Result: Articles 1 and 5, ideally only 5 (excluding article 1 because it's the main one).

Answer 1

I am not quite sure I understand why you didn't expect article 2 in your results, as it and article 1 both have tag 2. This below should still return article 2, so it may not be what you want, but it is the most straight forward "similarly tagged ranking" query I can think of:
SELECT b.*, COUNT(1) AS tagMatches
FROM articles AS a
INNER JOIN articles_tags AS aTags ON a.id=aTags.article_id
INNER JOIN articles_tags AS bTags
ON aTags.article_id<>bTags.article_id
AND aTags.tag_id = bTags.tag_id
INNER JOIN articles AS b ON bTags.article_id
WHERE a.url = ?
GROUP BY b.url
ORDER BY tagMatches DESC, b.title
;
Edit: This assumes articles cannot have the same tag more than once. If this is not the case, it will skew the rankings (but that might be favorable if the duplicated tags should have more weight).
Edit2: It is also worth noting, that * probably should not be used for final results; I just used it here for simplicity.

Answer 2

Your OR condition OR tags.id IN (articles_tags.tag_id)) fires on these rows:
INSERT INTO `articles_tags` (`article_id`, `tag_id`) VALUES
(1, 1),
...
(3, 1),
...,
(5, 1);
so, for me the result looks fine