Re: a union is always a join!
From: Brian Selzer <brian_at_selzer-software.com>
Date: Fri, 13 Mar 2009 19:34:27 -0400
Message-ID: <84Cul.18639$YU2.12549_at_nlpi066.nbdc.sbc.com>
>> "Walter Mitty" <wamitty_at_verizon.net> wrote in message
>> news:apltl.2309$%u5.1252_at_nwrddc01.gnilink.net...
>> 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
>> $20. Other cost records may exist for projects that he worked on after
>> the rate increase, and one should expect that a query of which projects
>> he worked on would return all of the projects, regardless of the labor
>> rate.
>> So something can appear different at different times yet still be the
>> same thing.
>> This poses a problem because keys are not necessarily permanent
>> identifiers. (I'm having trouble articulating my thought here because
>> there is more than one usage of the term, "key." I'm disinclined from
>> using "key value" because under an interpretation, a key value is a
>> mapping to a particular thing in the universe, that thing being the
>> output of the valuation function for the set of symbols for the
>> components in a tuple of the set of attributes that is the candidate key,
>> and it's possible for that same set of symbols to map to different things
>> at different times, or for different sets of symbols to map to the same
>> thing at different times. But it's unwieldy to say "sets of symbols for
>> the components in a tuple of the set of attributes that is the candidate
>> key" instead of just "keys.") The problem stems from how things in the
>> universe of discourse are identified, and that the scope of the
>> definition of a candidate key is any database and not all databases.
>> While a key may uniquely identify something in the context of its
>> containing database, that doesn't necessarily mean that that same key
>> uniquely identifies that same something at all databases in which it
>> appears.
Date: Fri, 13 Mar 2009 19:34:27 -0400
Message-ID: <84Cul.18639$YU2.12549_at_nlpi066.nbdc.sbc.com>
"paul c" <toledobythesea_at_oohay.ac> wrote in message news:ZXlul.17783$PH1.16918_at_edtnps82...
> Brian Selzer wrote:
>> "Walter Mitty" <wamitty_at_verizon.net> wrote in message
>> news:apltl.2309$%u5.1252_at_nwrddc01.gnilink.net...
>>> "Brian Selzer" <brian_at_selzer-software.com> wrote in message >>> news:eY2tl.9205$%54.7793_at_nlpi070.nbdc.sbc.com... >>>> "paul c" <toledobythesea_at_oohay.ac> wrote in message >>>> news:beZsl.15959$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 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.
> > I wish, at least once, you would give an answer that was shorter than the > question.
Ask me a question that has a simple answer, and I'll simply answer it. Received on Sat Mar 14 2009 - 00:34:27 CET