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

On 13 jul, 00:19, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> "Jan Hidders" <hidd..._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.

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.

• Jan Hidders