How to design a MySql Table for a Tag Cloud?

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有刺的猬
有刺的猬 2021-02-03 13:13

I have articles on my site, and I would like to add tags which would describe each article, but I\'m having problems with design mysql table for tags. I have two ideas:

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  • 2021-02-03 13:45

    yes, but you didn't answer my main question, how to get 3 most similar articles?

    Answer: Just look for the same tag ids in the merged table (tags_articles). Gather them and create a pattern.

    For example: Article 1 has tags: 1,2 Article 2 has tags: 2,3,4 Article 5 has tags: 6,7,2 Article 7 has tags: 7,1,2,3

    If you want the 3 most similar articles for article 1, you have to look for the tags 1,2. You'll find Article 7 is most similar and 2 and 5 have some similarities.

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  • 2021-02-03 13:46

    Generally, for this kind of many-to-many relationship, there are three tables :

    • The "article" table
      • primary key = id
    • The "tag" table
      • primary key = id
      • contains the data of each tag :
        • name, for example
    • A "tags_articles" table, which acts as a join table, and contains only :
      • id_article : foreign key that points to an article
      • id_tag : foreign key that points to a tag


    This way, there is no duplication of any tag's data : for each tag, there is one, and only one, line in the tag table.

    And, for each article, you can have several tags (i.e. several lines in the tags_articles table) ; and, of course, for each tags, you can have several articles.

    Getting a list of tags for an article, with this idea, is a matter of an additionnal query, like :

    select tag.*
    from tag
        inner join tags_articles on tag.id = tags_articles.id_tag
    where tags_articles.id_article = 123
    


    Getting the three "most similar" articles would mean :

    • select articles that have tags that the first article has
    • only use those which have the most important number of identical tags

    Not tested, but an idea might be something that would look like this :

    select article.id, count(*) as nb_identical_tags
    from article
        inner join tags_articles on tags_articles.id_article = article.id
        inner join tag on tag.id = tags_articles.id_tag
    where tag.name in ('php', 'mysql', 'erlang')
          and article.id <> 123
    group by article.id
    order by count(*) desc
    limit 3
    

    Basically, you :

    • select the articles ids for each tag that's present on your initial article
      • as there's an inner join, if an article in the DB has 2 tags that match the where clause, without the group by clause, there would be two lines for that article
      • of course, you don't want to re-select the article you already had -- which means it has to be excluded.
    • but, as you use group by article.id, there will be only one line per article
      • but you'll be able to use count, to find out how many tags each article has in common with the initial one
    • then, it's only a matter of sorting per number of tags, and getting only the third three lines.
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  • 2021-02-03 13:57

    First off, you'll want to use Pascal MARTIN's suggestion about the table design.

    As for finding similar articles, here's something to get you started. Given that @article_id is the article you want to find matches for, and @tag1, @tag2, @tag3 are the tags for that article:

    SELECT article_id, count(*)
    FROM tags_articles
    WHERE article_id <> @article_id
    AND tag_id IN (@tag1, @tag2, @tag3)
    GROUP BY article_id
    ORDER BY count(*) DESC
    LIMIT 3
    
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