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Re: A pk is *both* a physical and a logical object.

From: Brian Selzer <brian_at_selzer-software.com>
Date: Thu, 12 Jul 2007 22:19:08 GMT
Message-ID: <wNxli.21706$RX.8862@newssvr11.news.prodigy.net>

"Jan Hidders" <hidders_at_gmail.com> wrote in message news:1184270580.148732.271380_at_22g2000hsm.googlegroups.com...

> On 12 jul, 18:28, "David Cressey" <cresse..._at_verizon.net> wrote:

>> "Jan Hidders" <hidd..._at_gmail.com> wrote in message
>>
>> news:1184253165.108058.298260_at_n2g2000hse.googlegroups.com...
>>
>>
>>
>> > On 12 jul, 15:15, "David Cressey" <cresse..._at_verizon.net> wrote:
>> > > "Jan Hidders" <hidd..._at_gmail.com> wrote in message
>>
>> > >news:1184241371.515071.251680_at_k79g2000hse.googlegroups.com...
>>
>> > > > On 11 jul, 22:25, Cimode <cim..._at_hotmail.com> wrote:
>> > > > > Furthermore...
>> > > > > <<Technically a PK is *only* a physical implementation device,
>> > > > > not a
>> > > > > logical concept at all.>>
>>
>> > > > `When I use a word,' Humpty Dumpty said, in rather a scornful tone,
>> > > > `it means just what I choose it to mean -- neither more nor less.'
>>
>> > > > `The question is,' said Alice, `whether you can make words mean so
>> > > > many different things.'
>>
>> > > > `The question is,' said Humpty Dumpty, `which is to be master --
>> > > > that's all.'
>>
>> > > > ;-)
>>
>> > > > To answer the question, I think that is quite simple. As defined in
>> > > > the relational model it is a logical concept. As far as I know the
>> > > > SQL
>> > > > standard does not state that a PK implies an index (but I could be
>> > > > wrong) and then it is also there a logical concept. If it does
>> > > > imply
>> > > > an index then it is mixed concept because it has both logical and
>> > > > physical consequences.
>>
>> > > It was my understanding that the relational model defines keys, but
>> > > not
>> > > primary keys. That is, any candidate key is as much of a key as any
>> other.
>>
>> > Codd introduced the concept in his seminal paper, but yes, nowadays
>> > most researchers, including me, would agree that the notion doesn't
>> > make much sense at the logical level.
>>
>> > > On another subject, just what *is* the distinction between
>> > > "logical"
>> and
>> > > "physical". Over the decades since James Martin wrote on the
>> > > subject,
>> > > there seems to have been considerable drift in what the terms
>> > > actually
>> mean.
>>
>> > I don't know how Martin defined it, but in the context of databases it
>> > is relatively clearly defined in my opinion.
>>
>> Fine. And just what is that clear definition, if you please?
>
> At the logical level you describe the Universe of Discourse, the whole
> Universe of Discourse and nothing but the Universe of Discourse. :-)
>

Isn't there a definite separation between the Universe and the Discourse? I should think that those constraints that limit the course of the Discourse would be described at the logical level even though they clearly do not describe the Universe.

> -- Jan Hidders
> Received on Thu Jul 12 2007 - 17:19:08 CDT

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