Re: Multiple-Attribute Keys and 1NF

On Aug 29, 7:03 pm, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> "JOG" <j..._at_cs.nott.ac.uk> wrote in message
>
> news:1188393382.112445.286350_at_19g2000hsx.googlegroups.com...
>
>
>
> > 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
>
> >> >>news:1188327226.729673.127810_at_r34g2000hsd.googlegroups.com...
>
> >> >>>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
> >> >>green
> >> >>wire means earth" and "a yellow wire means earth". Pardon me for
> >> >>being
> >> >>dense.
>
> >> >>Clearly what we have here is not a domain of colors, but a domain of
> >> >>color
> >> >>codes, where a color code contains one or more colors, and maybe a
> >> >>"thick
> >> >>or thin" qualifier on each color.
>
> >> >>It's not clear to me why you need to able to query on simple colors,
> >> >>unless
> >> >>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
> >> >>more
> >> >>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
> >> >>can
> >> >>think of the character code for 'A' as being B64+B1. Now I could query
> >> >>on
> >> >>all the character codes without necessarily having an operator that
> >> >>would
> >> >>yield "all the codes that include B1".
>
> >> >>I think that the colors, as constituents of color codes, play the same
> >> >>role
> >> >>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.

I agree. I hold little stock with set based values so in RM I would go for the addition of colour-code foreign key.

But what if we weren't tied to a traditional relational schema and tweaked the system so it could allow propositions with more than one role of the same name without decomposing them. As Jan pointed out 'tuples' are no longer functions - they would be unrestricted binary relations (subsets of attribute x values). We could produce a comparatively simpler and less cluttered schema, predicate in a very similar manner as before, and with a few simple alterations could have an equally effective WHERE mechanism. My concern however would be the consequences to JOIN. Received on Wed Aug 29 2007 - 23:21:11 CEST