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From: "Brian Selzer" <brian@selzer-software.com>
Newsgroups: comp.databases.theory
References: <1184184150.157477.288010@q75g2000hsh.googlegroups.com>   <1184185507.133701.22450@m3g2000hsh.googlegroups.com>   <1184241371.515071.251680@k79g2000hse.googlegroups.com>   <tPpli.9982$qu4.3078@trndny06>   <1184253165.108058.298260@n2g2000hse.googlegroups.com>   <xEsli.8382$V35.2432@trndny03>   <1184270580.148732.271380@22g2000hsm.googlegroups.com>   <wNxli.21706$RX.8862@newssvr11.news.prodigy.net>   <1184283641.650361.251790@k79g2000hse.googlegroups.com>   <o1Dli.46147$5j1.41062@newssvr21.news.prodigy.net> <1184317017.026070.245210@22g2000hsm.googlegroups.com>
Subject: Re: A pk is *both* a physical and a logical object.
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Date: Fri, 13 Jul 2007 14:54:14 GMT
Xref: usenetserver.com comp.databases.theory:165764
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"Jan Hidders" <hidders@gmail.com> wrote in message 
news:1184317017.026070.245210@22g2000hsm.googlegroups.com...
> On 13 jul, 06:17, "Brian Selzer" <br...@selzer-software.com> wrote:
>> "Jan Hidders" <hidd...@gmail.com> wrote in message
>>
>> news:1184283641.650361.251790@k79g2000hse.googlegroups.com...
>>
>>
>>
>> > On 13 jul, 00:19, "Brian Selzer" <br...@selzer-software.com> wrote:
>> >> "Jan Hidders" <hidd...@gmail.com> wrote in message
>>
>> >>news:1184270580.148732.271380@22g2000hsm.googlegroups.com...
>>
>> >> > On 12 jul, 18:28, "David Cressey" <cresse...@verizon.net> wrote:
>> >> >> "Jan Hidders" <hidd...@gmail.com> wrote in message
>>
>> >> >>news:1184253165.108058.298260@n2g2000hse.googlegroups.com...
>>
>> >> >> > On 12 jul, 15:15, "David Cressey" <cresse...@verizon.net> wrote:
>> >> >> > > "Jan Hidders" <hidd...@gmail.com> wrote in message
>>
>> >> >> > >news:1184241371.515071.251680@k79g2000hse.googlegroups.com...
>>
>> >> >> > > > On 11 jul, 22:25, Cimode <cim...@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.
>>
>> > The course of the discourse is not part of the Universe of Discourse
>> > unless, of course, it is discussed in the discourse.
>>
>> > Or were you talking about dynamic constraints? Since those describe
>> > the allowed changes in the universe, much like physical laws describe
>> > the allowed changes of the physical universe, I think it is reasonable
>> > to say that they can be part of the description of a universe.
>>
>> I was thinking more along the lines of the difference between what can be
>> and what can be true.
>
> Very little, I imagine. :-)
>

{a, b, c} is different from {{a}, {b}, {c}}, yes?  A description of the 
universe is a description of what can be.  What can be true is a description 
of a description of what can be.

> So you meant static constraints? Why do you think those would not be
> part of the description of the UoD?
>

I was thinking more along the lines of relation schemata, which can be 
represented either by open sentences in first order logic (which if included 
as part of the description of the universe would mean that that description 
is incomplete) or by contingent sentences in quantified modal logic.  In 
either case an assignment is required to obtain what is actually true about 
their referents, and by extension, about the universe.

So, at the logical level, you describe not only the Universe of Discourse, 
but also what can be said about the Universe of Discourse.

> -- Jan Hidders
>