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

From: David Cressey <cressey73_at_verizon.net>
Date: Thu, 12 Jul 2007 19:21:37 GMT
Message-ID: <5bvli.11423$ZO4.10300@trndny05>

"Cimode" <cimode_at_hotmail.com> wrote in message news:1184260798.451810.60010_at_g4g2000hsf.googlegroups.com...
> On 12 juil, 18:25, "David Cressey" <cresse..._at_verizon.net> wrote:
> > "Cimode" <cim..._at_hotmail.com> wrote in message
> >
> > news:1184247582.799847.27210_at_o61g2000hsh.googlegroups.com...
> >
> >
> >
> > > On Jul 12, 2:15 pm, "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.
> >
> > > > > -- Jan hidders
> >
> > > > 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.
> > > In a relational perspective, the term *primary key* was first used by
> > > Codd to designate a specific unique identifier that allows to
> > > distinguish tuples.
> >
> > > > It was my understanding that certain schools of data management,
> > including
> > > > the SQL school, adopted the convention of naming one candidate key
as
> > > > primary key, and of making all FK references refer to that key,
where
> > > > possible. I can see, and use, that practice myself. But I can't
see
> > where
> > > > the relational model necessitates it.
> > > Who cares what SQL schools of *management* have to say about
> > > relational model?
> >
> > Please don't confuse "management" with "data managment".
> I think I misread your comment (did not mean to sound offending).

Yes, I think you did.

> I
> thought you were advocating that SQL schools of data management have
> any relevance anymore as to what should and should not be relationally
> sound. Quite frankly, I do not pay any attention to anything that is
> SQL related (except to make a living)..
>

SQL oriented people (I might include myself) will advocate declaring a primary key whenever a table is created, and making all FK references to the table via its primary key. That turns out to be of value even if the primary key concept is outside of the relational model of data. Received on Thu Jul 12 2007 - 14:21:37 CDT

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