Re: a union is always a join!
Date: Fri, 13 Mar 2009 05:13:29 GMT
Brian Selzer wrote:
> "Walter Mitty" <wamitty_at_verizon.net> wrote in message
>> "Brian Selzer" <brian_at_selzer-software.com> wrote in message >> news:eY2tl.9205$%54.7793_at_nlpi070.nbdc.sbc.com... >>> "paul c" <toledobythesea_at_oohay.ac> wrote in message >>> news:beZsl.15959$Db2.2243_at_edtnps83... >>>> Walter Mitty wrote: >>>> >>>>> ... I'm also going to suggest that what >>>>> Brain S. calls "oversimplification" is almost exactly what others call >>>>> "abstraction". I'm also going to suggest that without abstraction you >>>>> don't >>>>> get any independence, and without independence, you don't get much of >>>>> any >>>>> bang for the buck. That may be of zero theoretical importance, but >>>>> it's of >>>>> interest to me. >>>>> ... >>>> Walter, I'm with the many people who think phyaical and logical >>>> independence are of high importance, both theoretically and practically. >>>> But I'd say many of the nuances and implications of those haven't been >>>> explored much in print. Brain S as you call him regularly enters the >>>> realm of mysticism. I point this out not to correct him, but to warn >>>> newcomers here that he is not exactly swimming in the main stream of >>>> relational theory (to be fair, not many are, because the theory is often >>>> confused with past practice). I have a number of mystic acquaintances >>>> and I like them all, partly because they don't involve themselves in db >>>> theory and there is much in life for which mysticism offers the only >>>> comfortable clues. >>>> >>> 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
> 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
I wish, at least once, you would give an answer that was shorter than the question. Received on Fri Mar 13 2009 - 06:13:29 CET