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

"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.

> p
Received on Sat Jul 28 2007 - 01:06:37 CEST