Re: join and sort on 'best match'

hi pls tell me ----

if table *Item 3 : news, nature, greenpeace, whale has all clmn y v need
join ??*

*Ashish*

On 12/13/06, Ragnar <gnari(at)hive(dot)is> wrote:
>
> 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
>
>
>
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