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


"paul c" <toledobythesea@xxxxxxxx> wrote in message
news:2tyqi.11337$rX4.9610@xxxxxxxxxxxx
Brian Selzer wrote:
"paul c" <toledobythesea@xxxxxxxx> wrote in message
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David Cressey wrote:

"Brian Selzer" <brian@xxxxxxxxxxxxxxxxxxx> wrote in message
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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.


I repeat, what does it matter? If it happens to be a simple and precise
term, so what, eg., what is the point? What is the possible use? Eg.,
why would this notion ever matter to a dbms?


Given a choice between a key that permanently identifies individuals and a
key that contingently identifies individuals, which would you choose to be
the primary key, and thus the target of all foreign keys? Don't you see a
problem with a relation that represents the histories of a set of
individuals where those individuals are denoted only by a set of attributes
that contingently identify them?

But requiring that all key values permanently identify individuals
significantly limits the expressiveness of the model, cutting in half the
expressions that can be used to denote individuals, thereby reducing the
number of queries that can be formulated. Referring to my previous example,
a query such as "Which part has lot number 203 in location 22?" could not be
considered deterministic if the key, {lot_number, location}, were not
defined, and thus should be rejected as being formulated incorrectly.
Without that key, there can be more than one part with lot number 203 in
location 22, and you can't stuff a relation value into a tuple variable.

If somebody will please answer this question, I'll stop asking it!

p


.

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