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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 16:25:21 GMT
Message-ID: <RBsli.8337$475.7074@trndny04>

"Cimode" <cimode_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".

 So far, these people have proven mostly a deep
> ignorance of what the relational model implies. I stopped paying
> attention to them and feel much better since. A primary key (unique
> tuple identifier) is mandatory to establish that a relation is a
> relation. A relation body can *not* be having dupplicate tuples.
> Else it is not a relation.

>
> > 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.
> > Perhaps we have too many Humpty Dumpties in the field!

>

> In a relational perspective, one could view *logical* as dealing with
> defining the structure apects of relations, normalization, and what
> allows the system to make logical inferences to establish new defined
> facts.
>

> *Physical* refers to *how* such logical system is actually physically
> implemented. A total separation of the two layers is imperative to
> allow for a system to be called relational (independence between the
> two layers).
>

> To be coherent with such approach, my opinion is that primary key
> terminology should be avoided at all at physical level. Else one ends
> up making indexes and primary keys interchangeable. Direct image
> systems are the cause of such confusion (ORACLE, DB2, SQL Server)
>

> Regards...

> Received on Thu Jul 12 2007 - 11:25:21 CDT

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