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

"Brian Selzer" <brian_at_selzer-software.com> wrote in message news:1Uuqi.29231$2v1.33_at_newssvr14.news.prodigy.net...
>
> "paul c" <toledobythesea_at_oohay.ac> wrote in message
> news:wo1qi.7977$fJ5.772_at_pd7urf1no...
> > David Cressey wrote:
> >> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
> >> news:DjPpi.24618$Rw1.11254_at_newssvr25.news.prodigy.net...
> >>
> >>
> >>>This illustrates what happens when the only key on a relation schema
> >>
> >> permits
> >>
> >>>updates. It can't be determined if a new individual is being selected,
> >>>or
> >>>if the state of the current individual is now different.
> >>
> >>
> >> This is the point I have been trying to make for the past week or so.
> >> The
> >> mathematics of the relational data model don't, in this case,
> >> disambiguate
> >> two profoundly different scenarios in the real world the data purports
to
> >> describe.
> >> ...
> >
> > David, what does it matter?
> >
> > The user/audience can agree to disambiguate/interpret however it suits
> > their purpose/application.
> >
> > What is the possible usefulness of using this term "rigid" to describe a
> > key?
> >
>
> It is a simple and precise term that describes a class of identifiers.
> There are keys whose values identify a specific individual at all database
> values, and there are keys whose values identify a specific indivdual at
> some database values. For example, a relation that models an ordered set
> has two keys, one that represents names for elements and one that
represents
> positions for elements. Both meet all of the criteria for a candidate key
> (uniqueness and irreducibility), but only the one that represents names
> permanently identifies each element, since at different database values, a
> particular element may be in different positions.
>
The term "rigid" in this usage is new to me, with this discussion. Maybe its time to ask for a formal definition.

What is the distinction betweeb "rigid" and "immutable"? Does rigid imply immutable and vice versa? What is the benefit of a non rigid identifier? Do you call that a "limp" identifier?
Isn't immutability one of the criteria for a candidate key of a relation? Received on Sat Jul 28 2007 - 12:35:36 CEST