Re: Multiple-Attribute Keys and 1NF
Date: Wed, 29 Aug 2007 14:03:56 -0400
"JOG" <jog_at_cs.nott.ac.uk> wrote in message
> On Aug 29, 12:49 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>> JOG wrote:
>> > On Aug 29, 6:10 am, "David Cressey" <cresse..._at_verizon.net> wrote:
>> >>"JOG" <j..._at_cs.nott.ac.uk> wrote in message
>> >>>Okay, sure. But then to be able to query for green and yellow
>> >>>individually one must employ a further relation encoding two more
>> >>>propositions that state "'Green and yellow' contains 'Green'" and that
>> >>>"'Green and yellow' contains 'Yellow'" respectively. One then has a
>> >>>schema with two domains - one for the composites and one for
>> >>>individual colours (which is what I was inferring when I initially
>> >>>said a new one was being added).
>> >>It took me a while to realize that what you meant from your original
>> >>description was that
>> >>"a green and yellow wire means earth". I had thought you meant "a
>> >>wire means earth" and "a yellow wire means earth". Pardon me for
>> >>Clearly what we have here is not a domain of colors, but a domain of
>> >>codes, where a color code contains one or more colors, and maybe a
>> >>or thin" qualifier on each color.
>> >>It's not clear to me why you need to able to query on simple colors,
>> >>you need to decompose the color coding scheme into its constituent
>> >>parts for
>> >>some reason.
>> >>There are lot of code domains where each code is made up of a set of
>> >>primitive elements.
>> >>Perhaps a very relevant one might be "character code". If I have the
>> >>following primitive elements:
>> >>B1, B2, B4, B8, B16, B32, B64, B128
>> >>(which might be an odd way of labelling bits 0 through 7 of a byte), I
>> >>think of the character code for 'A' as being B64+B1. Now I could query
>> >>all the character codes without necessarily having an operator that
>> >>yield "all the codes that include B1".
>> >>I think that the colors, as constituents of color codes, play the same
>> >>as bits, as constituents of character codes. Do you agree?
>> > Yes. I mean no. No, yes. Gnngh ;)
>> > Ok, of course I understand your point - a wire can be viewed as having
>> > a colour code, which itself has constituent parts. But its just one
>> > interpretation right. I am still seeing a difference between the
>> > propositions:
>> > * There is a colour-code "yellow and green" that denotes "earth".
>> > * The casing of an earth wire features the colour yellow and the
>> > colour green.
>> > Its just like the difference between the propositions:
>> > * My office is B42
>> > * My office is on floor B, room 42.
>> > There are instances where I may well want to encode as the second
>> > proposition forms. And /if/ that were the case (iff), well 1NF is
>> > precluding me from doing this in terms of the wire example.
>> I disagree. You have already noted that 1NF allows this with exactly 2
>> relations (or with 1 relation and one or more operations on the color
>> code domain.)
> > True, I do see that, but it does so by requiring the invention of a > colour-code concept which isn't in the proposition "The casing of an > earth wire features the colour yellow and the colour green". >
You have to consider the entire relation value: what about the propositions (treating or exclusively, of course):
"The casing of a live wire features the colour brown or the colour red."
"The casing of a neutral wire features the colour blue or the colour black."
Write a predicate for the relation schema that when extentially quantified and extended yields a set of atomic formulae that implies all three of the propositions above. I think you'll find that the colour-code concept is in that predicate. Received on Wed Aug 29 2007 - 20:03:56 CEST