Re: a union is always a join!
Date: Sun, 15 Mar 2009 13:41:00 GMT
"Brian Selzer" <brian_at_selzer-software.com> wrote in message
> "Walter Mitty" <wamitty_at_verizon.net> wrote in message
>> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
>>> 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
> 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
> 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? Received on Sun Mar 15 2009 - 14:41:00 CET