Re: Multiple-Attribute Keys and 1NF

On Aug 30, 1:41 am, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> "JOG" <j..._at_cs.nott.ac.uk> wrote in message
>
> news:1188422471.161668.86550_at_r29g2000hsg.googlegroups.com...
>
>
>
> > 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.
>
> I'm not sure I understand what you are driving at. In the example you
> provided, it is the combinations of values from a simple domain that have
> significance, regardless of whether they're wrapped in a single attribute or
> not. To me it doesn't make sense to have multiple attributes with the same
> name--the attribute names correspond to free variables in a predicate: how
> could you assign multiple values to the same variable?

Well consider it this way. If I have the propositions:

The person named Jim speaks the language English The person named Jim speaks the language German The person named Brian speaks the language English

I have three propositions, and hopefully we'd agree there are two roles in these propositions: name and speaks_language. So in FOL I could write these propositions as:

[P1] Name(x, Jim) -> speaks_language(x, English)
[P2] Name(x, Jim) -> speaks_language(x, English)
[P3] Name(x, Brian) -> speaks_language(x, English)

Are we agreed up to there? If so then [P1] ^ [P2] gives us (via composition):
Name(x, Jim) -> speaks_language(x, English) ^ speaks_language(x, English)

and we are left with a sentence that has two distinct roles, one of which appears twice. All of this sort of thinking has been driven by a distaste us having to add a magic 'header' component to a relation (probably as a consequence of reading pascal's work), and the desire to bring roles back into the equation.

> But you can
> certainly assign a set of values to a variable that expects a set of values,
> since a set is a value! And you can certainly have a predicate with free
> variables that range over relations and free variables that range over
> individuals--it's just that the predicate is no longer first order.
Received on Thu Aug 30 2007 - 13:27:14 CEST