Re: Multiple-Attribute Keys and 1NF

On Aug 30, 1:44 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> JOG wrote:
> > On Aug 30, 1:42 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
> >>JOG wrote:
>
> >>>>Write a predicate for the relation schema that when extentially quantified
> >>>>and extended yields a set of atomic formulae that implies all three of the
> >>>>propositions above. I think you'll find that the colour-code concept is in
> >>>>that predicate.
>
> >>>I agree. I hold little stock with set based values so in RM I would go
> >>>for the addition of colour-code foreign key.
>
> >>>But what if we weren't tied to a traditional relational schema and
> >>>tweaked the system so it could allow propositions with more than one
> >>>role of the same name without decomposing them. As Jan pointed out
> >>>'tuples' are no longer functions - they would be unrestricted binary
> >>>relations (subsets of attribute x values). We could produce a
> >>>comparatively simpler and less cluttered schema, predicate in a very
> >>>similar manner as before, and with a few simple alterations could have
> >>>an equally effective WHERE mechanism. My concern however would be the
> >>>consequences to JOIN.
>
> >>What would you offer in place of the RM's logical identity.
>
> > Nothing. I am utterly convinced by Date et al's arguments in favour of
> > logical identity. (Why would I need to replace it?) I just wanna model
> > propositions, and they are always identified by their contents.
>
> In: {{(Color: green), (Color: yellow), (Type: earth)}}
>
> What provides logical identity?

I may be misunderstanding you, but let me take a stab. The identity of any set of course lies in its elements (i.e. in this of a single propositions, the ordered pairs). Given we know Colors are the antecedents in the proposition we are modelling, this has to be been defined in the collectivizing predicate for the whole collection of rows. We also know therefore there may not exist another set of pairs containing the same Colors, so we can identify the whole proposition through examination of just those roles. All works just as per normal in RM. Is this what you meant? Received on Thu Aug 30 2007 - 15:27:45 CEST