Re: A pk is *both* a physical and a logical object.
Date: Thu, 12 Jul 2007 15:12:45 -0000
On 12 jul, 15:15, "David Cressey" <cresse..._at_verizon.net> wrote:
> "Jan Hidders" <hidd..._at_gmail.com> wrote in message
> > 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.
> 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