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
