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

From: David Cressey <cressey73_at_verizon.net>
Date: Thu, 12 Jul 2007 16:28:13 GMT
Message-ID: <xEsli.8382$V35.2432@trndny03>

"Jan Hidders" <hidders_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? Received on Thu Jul 12 2007 - 11:28:13 CDT