Re: a union is always a join!

"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. Received on Wed Mar 11 2009 - 07:44:17 CET