How to implement tagging system similar to SO in php/mysql?

Question

I\'m coding a website in PHP/MySQL and I\'d like to implement a similar to stackoverflow tagging engine. I have 3 relevant tables in DB: 1. Items 2. Tags 3. ItemTagMap (maps tag

Answer 1

Assuming:

• Item (id);
• Tag (id, name) with index on name;
• ItemTag (item_id, tag_id).

then:

SELECT t.name
FROM Tag t
WHERE EXISTS (SELECT 1 FROM ItemTag WHERE item_id = 1234)
ORDER BY t.name

Nothing intensive about that. This is similar but my guess is it would be slower:

SELECT t.name
FROM Tag t
WHERE t.id IN (SELECT tag_id FROM ItemTag WHERE item_id = 1234)
ORDER BY t.name

This can be done as a join as well:

SELECT DISTINCT t.name
FROM Tag t
JOIN ItemTag i WHERE i.tag_id = t.id
WHERE i.item_id = 1234
ORDER BY t.name

I think the first one will be faster but as is always the case with SQL, it's worth testing (on a sufficiently sized data set).

The above have been done to list the tags for a single item. You want a composite set of tags for search results. That's not difficult from the above but it depends on how you get your search results.