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

On 13 jul, 06:17, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> "Jan Hidders" <hidd..._at_gmail.com> wrote in message
>
> news:1184283641.650361.251790_at_k79g2000hse.googlegroups.com...
>
>
>
> > 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.
>
> I was thinking more along the lines of the difference between what can be
> and what can be true.

Very little, I imagine. :-)

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

• Jan Hidders