join and sort on 'best match'

Hi Everybody,

I have been breaking my head on the following problem: how to join 2
tables and sort the results on the best match.

explanation:

- there are 3 tables, items, tags and items_tags. The items_tags table
links items to tags.
- I have one item which has certain tags, and I want to look up all the
other items that have those tags as well
- results should be sorted and presented by 'best match': first all the
items that have 3 tags in common, then 2 and last 1

example:

Item 1 : news, nature, greenpeace
Item 2 : news, nature
Item 3 : news, nature, greenpeace, whale

Item 1 and Item 3 are the best match.

So far, the SQL I came up wiht looks like:

SELECT id, COUNT(items_tags.item_id) AS quantity
FROM items JOIN items_tags ON items_tags.item_id = items.id
WHERE id in (select item_id from items_tags where tag_id in (select
tag_id from items_tags where item_id=?))
GROUP BY items_tags.item_id,id
ORDER BY quantity DESC

note: the '?' in the query represents the dynamic part: I have 1 item
and I want to look up matching items.

To me, this query means the following:

- get all items that have tags, the 'JOIN', and count the tags, but only
those that match on the same tags, the 'WHERE'
- then show them

I thought I had found the solution (my test cases worked), but I now
find cases that should be found by the query but are not.

Can anyone please help me?

Dirk