Re: Multiple-Attribute Keys and 1NF

On Aug 31, 1:58 pm, "David Cressey" <cresse..._at_verizon.net> wrote:
> "JOG" <j..._at_cs.nott.ac.uk> wrote in message
>
> news:1188556656.192653.305160_at_r23g2000prd.googlegroups.com...
>
> > 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.)
>
> 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.

I think there is a lot of truth in what you say. Codd was impressive in his ability to be theoretical and pragmatic. His success comes in part I believe to his social awareness of the current climate in which he was working. Received on Fri Aug 31 2007 - 15:28:40 CEST