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

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. On the other hand I know some data modelling researchers that will insist that when maping your ER / ORM / FDM / whatever model to a relational database you are mapping a conceptual model to a physical model. Who am I to deny them their terminology. :-)

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

That is certainly a fact, but I'm not sure if I'm not more worried by the Dumpty Humpties, i.e., those that will insist that there is only one true meaning of a word and every attempt to redefine it is no less than sacrilege. My guess is that it is the fundamentalists that we need to worry about more.

• Jan Hidders