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

From: paul c <toledobythesea_at_oohay.ac>
Date: Sat, 28 Jul 2007 03:10:54 GMT
Message-ID: <2tyqi.11337$rX4.9610@pd7urf2no>

Brian Selzer wrote:

> "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.
> 

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?

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

p Received on Fri Jul 27 2007 - 22:10:54 CDT