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

"Brian Selzer" <brian_at_selzer-software.com> wrote in message news:7bqni.22449$RX.18708_at_newssvr11.news.prodigy.net...
>
> "David Cressey" <cressey73_at_verizon.net> wrote in message
> news:Cmhni.6545$Gx5.2883_at_trndny02...
> >
> > "Brian Selzer" <brian_at_selzer-software.com> wrote in message
> > news:oXdni.23174$Rw1.4623_at_newssvr25.news.prodigy.net...

> >
> > widgets do not have identity in this scheme.
>
> Ah, but they do. {lot_number, location} is a key. You could also have a
> relation widgit_part_numbers {lot_number, location, customer,
> customer_part_number} that references widgits. What widgits don't have is
> rigid designation. That's the problem: it cannot be determined if a
> referent at one possible database value is the same as a referent at
> another.

I think you and I mean something different by "identity".

lot_number is not an attribute of a widget, if I understand your subject matter correctly. It's an attribute of a lot, and a widget is a member of a lot.

location is not an attribute of a widget either. It's an attribute of a location (sorry about overloading the word "location"), and a widget can be stored at a location.

You can't identify an entity solely by listing attributes of other entities to which the entity in question has (possibly temporal) relationships.

Hence {lot_number, location} can't be the identity of a widget. The relation isn't really about widgets. It's about widget placements. And, if you can store more than one widget from a lot in a location, it isn't even a relation; it's a bag. Received on Wed Jul 18 2007 - 19:56:35 CEST