Re: a union is always a join!

From: Brian Selzer <brian_at_selzer-software.com>
Date: Sun, 15 Mar 2009 21:26:33 -0400
Message-ID: <dVhvl.26343$ZP4.16548_at_nlpi067.nbdc.sbc.com>


"Walter Mitty" <wamitty_at_verizon.net> wrote in message news:Mz7vl.234$6%.181_at_nwrddc01.gnilink.net...
>
> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
> news:65Jtl.9343$%54.435_at_nlpi070.nbdc.sbc.com...
>>
>> "Walter Mitty" <wamitty_at_verizon.net> wrote in message
>> news:apltl.2309$%u5.1252_at_nwrddc01.gnilink.net...
>>>
>>> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
>>> news:eY2tl.9205$%54.7793_at_nlpi070.nbdc.sbc.com...
>>>> Mysticism. If accepting that the universe of discourse contains things
>>>> and that at different times a thing can differ in appearance yet still
>>>> be the same thing means that I'm a mystic, then I'm guilty as charged.
>>>
>>> What difference does it make whether it's the same thing or a different
>>> thing?
>>
>> If an employee worked 50 hours on a project and his labor rate is $20 per
>> hour, then it cost $1000 to have him work on the project, right? WRONG!
>> The employee's labor rate /is/ $20 per hour, but that doesn't mean that
>> it /had been/ $20 per hour during the time that he worked on the project.
>> At that time his labor rate might have been $18 per hour or may even have
>> changed part way through the project. So the record of cost must not
>> contain just which project, which employee and how many hours, but also
>> at which labor rate or rates the work was performed. But the employee is
>> still the same employee even though his labor rate changed from $18 to
>> $20. Other cost records may exist for projects that he worked on after
>> the rate increase, and one should expect that a query of which projects
>> he worked on would return all of the projects, regardless of the labor
>> rate.
>>
>> So something can appear different at different times yet still be the
>> same thing.
>>
>> This poses a problem because keys are not necessarily permanent
>> identifiers. (I'm having trouble articulating my thought here because
>> there is more than one usage of the term, "key." I'm disinclined from
>> using "key value" because under an interpretation, a key value is a
>> mapping to a particular thing in the universe, that thing being the
>> output of the valuation function for the set of symbols for the
>> components in a tuple of the set of attributes that is the candidate key,
>> and it's possible for that same set of symbols to map to different things
>> at different times, or for different sets of symbols to map to the same
>> thing at different times. But it's unwieldy to say "sets of symbols for
>> the components in a tuple of the set of attributes that is the candidate
>> key" instead of just "keys.") The problem stems from how things in the
>> universe of discourse are identified, and that the scope of the
>> definition of a candidate key is any database and not all databases.
>> While a key may uniquely identify something in the context of its
>> containing database, that doesn't necessarily mean that that same key
>> uniquely identifies that same something at all databases in which it
>> appears.
>>
> Aren't you just saying that a variable does not record its own history?
>

No. I'm saying that the tuple in one database that maps to a particular thing in the universe of discourse can be a different set of components from the tuple in the succeeding database that maps to that same thing. In fact, any or even all of the components can be different--including key components. Received on Mon Mar 16 2009 - 02:26:33 CET

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