Re: Multiple-Attribute Keys and 1NF
Date: Sat, 01 Sep 2007 10:21:06 -0000
On Sep 1, 2:31 am, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> "David Cressey" <cresse..._at_verizon.net> wrote in message
> > "JOG" <j..._at_cs.nott.ac.uk> wrote in message [Quoted]
> >> Well I've never suggested multiple values contained in a collection.
> >> But yes as I said, multiple roles does break the guaranteed access
> >> rule. My question is now (in the continuuing hunt for the theory
> >> behind 1NF) is why on earth would that be a problem? I don't see any
> >> affect on the relational algebra.
> > I honestly think that the impetus behind "normalization" in the Codd 1970
> > paper is more of a stopgap than a theory. (I'm not familiar with the
> > 1969
> > paper, and I only read the 1970 paper after I began participating in the
> > discussions in c.d.t.) In the 1970 paper, Codd suggests that it may be
> > worthwhile to consider the subset of schemas that contain only atomic
> > attributes. (He didn't use the word "schemas", but I hope I can use it
> > without introducing confusion.)
> The whole point of 1NF boils down to the following two sentences from the
> June, 1970 article: "The adoption of a relational model of data, as
> described above, permits the defelopment of a universal data sublanguage
> based on an applied predicate calculus. A first-order predicate calculus
> suffices if the collection of relations is in normal form." Therefore, the
> impetus behind "normalization" is to model the universal data sublanguage
> after a first-order logic rather than some higher order logic.
That's a great quote - probably the best reasoning i've seen for 1NF. Thanks.
> > He pointed out that such a restriction did not thereby reduce the
> > expressiveness to the system, in that for every unnormalized schema,
> > there
> > existed an equivalent normalized schema. "normalized" in the 1970 paper
> > is
> > called 1NF in later writings, once further normal forms were discovered.
> > There is one other piece of the 1NF definition in the 1970 paper, the "no
> > duplicates rule". The no duplicates rule has to do with the
> > representation
> > of a relation, and not with a relation itself. Codd imagined (correctly)
> > that the first relational database systems would use records to represent
> > tuples and (virtual) arrays of records to represent relations. In a
> > relation, there is no such thing as "a tuple appearing twice". However,
> > in
> > an array of records, there is such a thing as two of the records having
> > identical contents. Codd ruled that out as a practical stop gap, in
> > order
> > to prevent the implementations from diverging from the properties of
> > mathematical relations in an unnecessary and harmful way. This is my
> > reading of the 1970 paper, in regard to 1NF theory.
> > There's a connection between the "atomic values" rule and the "no
> > duplicates
> > rule", at the implementation level.
> > consider the following fact:
> > Jack speaks English and German.
> > Let's say we are about to include this fact in a relation stored somewhere
> > in a relational database, and that one of the columns of a relational
> > table
> > is "set of languages spoken".
> > Further, let's say that there is already a tuple in the relation with the
> > following fact stored:
> > Jack speaks German and English.
> > As a practical matter, in terms of the representation of data inside a
> > database, it can be extraordinarily difficult to ascertain that these two
> > propositions, together, violate the "no duplicates rule"
> > Notice that my focus has been entirely on the implementation, and not on
> > the relational algebra itself. With regard to the relational algebra
> > itself, I believe your understanding is correct.
> > So what the heck are implementation oriented issues doing in the 1970
> > paper?
> > I believe Codd wanted to get across two main ideas: building a system for
> > relational databases would be a good idea. And building such a system was
> > also feasable. It's for this second reason that I believe Codd added some
> > material that is primarily about implementation, rather than about the
> > power
> > of relational algebra itself.
> > This is my insight, such as it is. I hope it helps.
Received on Sat Sep 01 2007 - 12:21:06 CEST