Subject: Re: Coalesce/Join/Entries may not exist.
From: Rod Taylor <rbt@rbt.ca>
To: jtx@hatesville.com
Cc: pgsql-sql@postgresql.org
In-Reply-To: <000101c32e7e$b4353a10$0300a8c0@theplague>
References: <000101c32e7e$b4353a10$0300a8c0@theplague>
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Date: 10 Jun 2003 07:57:10 -0400

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On Mon, 2003-06-09 at 08:00, James Taylor wrote:
> I've got three tables, I'll shorten the columns down just so you get the
> idea:
>=20
> lists
> -----------
> id|order_id
>=20
> list_results
> ------------
> id|lid|total
>=20
> orders
> ------------
> id|max
>=20
> All of the columns are int's.  What I'm trying to do is something like:
>=20
> select (o.max-coalesce(sum(lr.total),0)) from orders o,list_results lr
> where lr.l_id in (select
>    l.id from lists l, orders o where l.order_id=3DX and o.id=3Dl.order_id)
> group by o.max
>=20
> This would, in theory, return a number which should be
> o.total-sum(lr.total)
>=20
> The problem is, there may not be any data in list_results OR lists
> regarding the order ID.  If data from list_results.total exists, and is
> referencing lists.id, which in turn is referencing orders.id through
> lists.order_id, return o.max-lr.total.  If data from list_results or
> lists DOESN'T exist, I would just want to go ahead and return
> orders.max.  I was hoping the coalesce would be able to do this, but it
> doesn't.  The subquery is in there because frankly I'm not sure how to
> do multiple left joins, which I think would have to exist.  The easy way
> out for me here I think would be to make list_results.order_id and leave
> lists out of it, but then I'd have redundant data in two tables.  Any
> suggestions on this one?

Multiple left outer joins?

FROM <tab>
LEFT OUTER JOIN <tab2> USING (<column>)
LEFT OUTER JOIN <tab3> USING (<column>)
WHERE ...

In your case,=20


SELECT (o.max - coalesce(sum(lr.total), 0))
  FROM orders
LEFT OUTER JOIN lists ON (orders.id =3D lists.order_id)
LEFT OUTER JOIN list_results ON (lists.id =3D list_results.l_id)
GROUP BY o.max

But I'm not sure if that accomplishes what you're looking for or not.

--=20
Rod Taylor <rbt@rbt.ca>

PGP Key: http://www.rbt.ca/rbtpub.asc

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