Re: Multiple-Attribute Keys and 1NF
Date: Wed, 29 Aug 2007 20:41:21 -0400
Message-ID: <SmoBi.1348$z_5.1227_at_nlpi069.nbdc.sbc.com>
"JOG" <jog_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? 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 - 02:41:21 CEST