Re: Multiple-Attribute Keys and 1NF

On Aug 30, 5:00 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> JOG wrote:
> > On Aug 30, 2:55 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
> >>JOG wrote:
>
> >>>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?
>
> >>I haven't got a clue what you said.
>
> > I just regurgitated leibniz identity.
>
> >>In the RM, every value is uniquely
> >>identifiable by the combination of relation name, attribute name and any
> >>candidate key value. That's logical identity as it was originally
> >>spelled out.
>
> >>Two values above have the same attribute name.
>
> > Now you've lost me. A "value" is not identifiable by its relation name
> > and attribute name. This makes no sense to me. Where in predicate
> > logic does that come from? A value is just a value. It is identifiable
> > in its own right as being an individual from a domain.
>
> I mispoke. "Any value represented in a relvar"

Well it is still just a value whether its in a relvar or not - it needs no extra identity. A database table is just a set of propositions. A proposition is encoded as a set of attribute-value pairs. That's it surely?

Any notion of identity is as defined by set theory.

>
> > An individual piece of /data/ however (which is perhaps what you mean
> > by a value) has an identity made up of a combination of an attribute
> > name and a corresponding value. One needs both to identify the data
> > item. A proposition in turn is identifiable by its contents, which is
> > a set of those data items. Regards, J.
>
> I repeat: two pieces of data have the same name, Color.

Well no - a piece of data doesn't have a 'name' does it? It's just a combination of attribute and value. The number-7. name-Fred. colorred.  A datum's identity is defined by the /combination/ of these two parts, and that alone - not by a label, or an alias, or an OID (as I'm sure you'd agree).

And if two datum share one of these parts (the attribute component) , well so what - they are still identifiably different things. I'm scratching my head to see the problem you are envisaging here. Received on Thu Aug 30 2007 - 18:58:28 CEST