On mið, 2006-12-13 at 10:26 +0100, Dirk Griffioen wrote:
> I have been breaking my head on the following problem: how to join 2
> tables and sort the results on the best match. ^^^^^^^^^^^^^
> - 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
looks to me like you want to join:
items->item_tags->tags->item_tags->items
so the basic select is:
SELECT * FROM items AS i1 JOIN items_tags AS it1 ON (it1.item_id = i1.id) JOIN tags AS t ON (t.tag_id =
it1.tag_id) JOIN items_tags AS it2 ON (it2.tag_id = t.tag_id) JOIN items AS i2 ON (i2.id = it2.item_id) WHERE
i1.id=?
> - results should be sorted and presented by 'best match': first all
> the items that have 3 tags in common, then 2 and last 1
this would be:
SELECT i1.id,i2.id,COUNT(*) as quantity FROM items AS i1 JOIN items_tags AS it1 ON (it1.item_id = i1.id)
JOINtags AS t ON (t.tag_id = it1.tag_id) JOIN items_tags AS it2 ON (it2.tag_id = t.tag_id) JOIN items AS i2
ON(i2.id = it2.item_id) WHERE i1.id=? GROUP by i1.id,i2.id ORDER BY quantity DESC
> 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.
if this does not work, please provide us with a counter example.
gnari
