Subject: Re: join and sort on 'best match'
From: Ragnar <gnari@hive.is>
To: Dirk Griffioen <dirk@code-shop.com>
Cc: pgsql-sql@postgresql.org
In-Reply-To: <457FC751.6080104@code-shop.com>
References: <457FC751.6080104@code-shop.com>
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Date: Wed, 13 Dec 2006 10:29:47 +0000
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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)
       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=?
  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