Re: Multiple-Attribute Keys and 1NF
From: paul c <toledobythesea_at_oohay.ac>
Date: Tue, 28 Aug 2007 16:37:25 GMT
Message-ID: <9bYAi.100937$rX4.78680_at_pd7urf2no>
> ... I seem to remember a while back there was a
> discussion involving Marshall and a few others considering situations
> where a nested relation was /necessary/ (I need to have a dig for it),
> and it didn't sit comfortably then.
> ...
Date: Tue, 28 Aug 2007 16:37:25 GMT
Message-ID: <9bYAi.100937$rX4.78680_at_pd7urf2no>
JOG wrote:
> On Aug 28, 5:05 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>> JOG wrote:
> ... I seem to remember a while back there was a
> discussion involving Marshall and a few others considering situations
> where a nested relation was /necessary/ (I need to have a dig for it),
> and it didn't sit comfortably then.
> ...
My old favourite is the relation that shows combinations, say with two attributes that make a composite key. It's a bit obscure maybe, but an example query is "combinations of parts that have ever been shipped". I think the only way to answer it *verbatim* is to group {shipment#, part#} on part# and then project away shipment#. I seem to recall that Date uses a "catalog" example involving different sets of candidate keys for a single relation.
p Received on Tue Aug 28 2007 - 18:37:25 CEST