Re: Multiple-Attribute Keys and 1NF
Date: Thu, 30 Aug 2007 14:33:07 -0300
Message-ID: <46d6ff06$0$4063$9a566e8b_at_news.aliant.net>
JOG wrote:
> 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. > >>>>item. A proposition in turn is identifiable by its contents, which is
>>>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
>>>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. color- > red. 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).
No, I don't agree. I suggest you see the definition of Logical Identity in Codd's 12 rules. Received on Thu Aug 30 2007 - 19:33:07 CEST