Question

So, I have a database with a table called artcles, and also a table called article tags. When a user views an article, I want to query up to five articles that have tags similar to the one that is being viewed. Here are my two tables:

CREATE TABLE `articles` (
  `article_id` int(15) NOT NULL AUTO_INCREMENT,
  `parent_id` int(15) NOT NULL,
  `author_id` int(15) NOT NULL,
  `title` text NOT NULL,
  `content` text NOT NULL,
  `date_posted` text NOT NULL,
  `views` int(15) NOT NULL,
  `preview` text NOT NULL,
  `status` tinyint(1) NOT NULL,
  `modified_date` text NOT NULL,
  PRIMARY KEY (`article_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1;

CREATE TABLE `article_tags` (
  `tag_id` int(15) NOT NULL AUTO_INCREMENT,
  `article_id` int(15) NOT NULL,
  `keyword` varchar(250) NOT NULL,
  PRIMARY KEY (`tag_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1;

I’ve tried writing my own queries, but they never seem to work. I would like to use joins in the query instead of resorting to using CSV’s and LIKE. Here’s the query that I have so far:

SELECT A2.article_id, count(A2.article_id) AS matches
FROM article_tags AS A1 JOIN article_tags ON (A1.keyword = A2.keyword AND 1.article_id != A2.article_id)
JOIN articles ON (A2.article_id = A.article_id) AS A
WHERE A1.article_id = 1
GROUP BY A2.article_id
ORDER BY matches DESC
LIMIT 5"

This is my updated query:

$query = "
            SELECT t2.article_id, count(t2.keyword) AS matches
            FROM article_tags t1
            JOIN article_tags t2 ON (t1.keyword = t2.keyword AND t1.article_id != t2.article_id)
            WHERE t1.article_id = ".$article_id."
            GROUP BY t2.article_id
            ORDER BY matches DESC
            LIMIT 5";

This is the result of dumping the array with var_dump

array
  0 => 
    array
      'article_id' => string '2' (length=1)
      'matches' => string '1' (length=1)

$query = "
            SELECT t2.article_id, count(t2.keyword) AS matches
            FROM article_tags t1
            JOIN article_tags t2 ON (t1.keyword = t2.keyword AND t1.article_id != t2.article_id)
            WHERE t1.article_id = ".$article_id."
            GROUP BY t2.article_id
            ORDER BY matches DESC
            LIMIT 5";

        if($query = $this->db->query($query)){

            if($query->num_rows() > 0){

                foreach($query->result_array() as $id => $article){

                    $articles[$id] = $this->fetch_article($article['article_id']);

                }

            } else {

                $articles = array();

            }

        } else {

            $articles = array();

        }

        return $articles;

    }

Answer 1

Basically your thinking is correct – make a self JOIN on article_tags table. There are something that you should improve:

• COUNT tag_id instead of article_id, since you want to sort articles by relevance, and the count of matched tags indicates the relevance.
• Join on tag_id instead of keyword. Join on non-indexed column will be a performance issue.
• Do not use a != in JOIN condition for the reason of performance. Just get all related articles and simply remove the most related one, which should be the current article itself
• Join on articles is not necessary, for the reason of performance. You don’t need the articles themselves; just do a simple SELECT on articles after you get ids of the 5 related articles.

So the answer could be something like this:

SELECT
    A2.article_id, count(A2.tag_id) AS matches
FROM 
    article_tags A1 
JOIN
    article_tags ON A1.tag_id=A2.tag_id
WHERE
    A1.article_id = 1
GROUP BY
    A2.article_id
ORDER BY
    matches DESC
LIMIT 6   -- instead of 5, because the first result would be the current article

You should get an array with 6 ids, and just remove the first one, then do a SELECT(e.g. in python):

article_ids = article_ids[1:]
articles = cursor.execute(
    "SELECT * FROM articles WHERE article_id IN (%s)" % ",".join(article_ids)
)