Re: A pk is *both* a physical and a logical object.
From: Brian Selzer <brian_at_selzer-software.com>
Date: Fri, 13 Jul 2007 04:17:24 GMT
Message-ID: <o1Dli.46147$5j1.41062_at_newssvr21.news.prodigy.net>
>> "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.
Date: Fri, 13 Jul 2007 04:17:24 GMT
Message-ID: <o1Dli.46147$5j1.41062_at_newssvr21.news.prodigy.net>
"Jan Hidders" <hidders_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.
> -- Jan Hidders
>
Received on Fri Jul 13 2007 - 06:17:24 CEST