Алгоритм, который ищет связанные объекты на основе общих тегов

I didn't have time to optimize the WHERE IN() clause which is slow as death. I also didn't both to include indexes or specify the engine or collations but this should hopefully do the trick. Please point out any mistakes as I have not actually tested this sloppy code:

CREATE TABLE question (
    id INT(11) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
    title VARCHAR(255) NOT NULL,
    question MEDIUMTEXT
);

CREATE TABLE question_rel_tag (
    id INT(11) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
    question_id INT(11) UNSIGNED NOT NULL,
    tag_id INT(11) UNSIGNED NOT NULL
);

CREATE TABLE tag (
    id INT(11) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(20) NOT NULL
);

SELECT question.id, question.title, question.question, count(tag.id) AS `count`
FROM question
INNER JOIN question_rel_tag ON (question.id = question_rel_tag.question_id)
INNER JOIN tag ON (question_rel_tag.tag_id = tag.id)
WHERE question.id != YOUR_QUESTION_ID_HERE
AND tag.id IN 
    (SELECT tag.id FROM question
    INNER JOIN question_rel_tag ON (question.id = question_rel_tag.question_id)
    INNER JOIN tag ON (question_rel_tag.tag_id = tag.id)
    WHERE question.id = YOUR_QUESTION_ID_HERE)
GROUP BY tag.id
ORDER BY `count` DESC

Suppose I have the following:

A table of taggable entities:

Taggable

id, stuff

A table of tags:

Tag

id, tagName

A join table of what tags are associated with what Taggables

TagInfo

taggableId tagId

Then :

SELECT COUNT(Taggable_1.id) AS score, Taggable_1.id FROM dbo.Taggable INNER JOIN dbo.TagInfo ON dbo.Taggable.id = dbo.TagInfo.taggableId INNER JOIN dbo.TagInfo TagInfo_1 ON dbo.TagInfo.tagId = TagInfo_1.tagId INNER JOIN dbo.Taggable Taggable_1 ON TagInfo_1.taggableId = Taggable_1.id WHERE (dbo.Taggable.id = 1) AND (Taggable_1.id <> 1) GROUP BY Taggable_1.id

would join the tag-join table against itself (for the queried question id; 1 in the above sql) and count up the results for a score.

Assuming a table Questions with a primary key column Id, a table Tags with a primary key column Id, too, and a join table QuestionTags with a composite primary key QuestionId and TagId referencing the primary keys of the two former tables, the following query will give the desired result (in SQL Server 2005).

SELECT q1.Id AS Id1, q2.Id AS Id2,
   (SELECT COUNT(*) FROM QuestionTags qt1 INNER JOIN QuestionTags qt2
    ON qt1.QuestionId = q1.Id AND qt2.QuestionId = q2.Id AND qt1.TagId = qt2.TagId) AS TagCount
FROM Questions q1 INNER JOIN Questions q2 ON q1.Id < q2.Id
ORDER BY TagCount DESC

This can be improved to the following.

SELECT qt1.QuestionId AS Id1, qt2.QuestionId AS Id2, COUNT(*) AS TagCount
FROM QuestionTags qt1 INNER JOIN QuestionTags qt2
ON qt1.QuestionId < qt2.QuestionId AND qt1.TagId = qt2.TagId
GROUP BY qt1.QuestionId, qt2.QuestionId
ORDER BY TagCount DESC

select *
     , count(t.*) as matched_tag_count
  from questions q
  join tags_to_questions t
    on t.question_id = q.id
   and t.tag_id in (select tag_id
                      from tags_to_questions
                     where question_id = ?)
group
    by q.id
order
    by matched_tag_count desc