Re: A pk is *both* a physical and a logical object.
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Sat, 28 Jul 2007 01:13:56 -0300
Message-ID: <46aac255$0$4024$9a566e8b_at_news.aliant.net>
>> "paul c" <toledobythesea_at_oohay.ac> wrote in message
>> news:wo1qi.7977$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.
Date: Sat, 28 Jul 2007 01:13:56 -0300
Message-ID: <46aac255$0$4024$9a566e8b_at_news.aliant.net>
paul c wrote:
> Brian Selzer wrote: >
>> "paul c" <toledobythesea_at_oohay.ac> wrote in message
>> news:wo1qi.7977$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!
I think it has the most meaning in reference to tools, of which Selzer is one. Received on Sat Jul 28 2007 - 06:13:56 CEST