Re: Interesting article: In the Beginning: An RDBMS history
Date: Mon, 10 Apr 2006 14:13:35 +0300
"David Cressey" <dcressey_at_verizon.net> wrote in message
> "mAsterdam" <mAsterdam_at_vrijdag.org> wrote in message
> > As soon as it does change (e.g. add/scratch an
> > attribute/column/field/whatever horizontal thingy)
> > the /number/ references have to be re-examined whereas the /name/
> > simply continues to refer to the correct (as
> > correct as it was to begin with) data.
> The above is true if the numbers are determined by a position in a list.
> the numbers are simply assigned
> sequentially, as if they were "names", you could alter relational tables
> or relations without regard to the
> "column numbers". However, since attributes in a relation are written
> by POSITION, and not merely by numerical name, your point is valid with
> regard to mathematical relations.
> Going back to the original question which was (roughly) "what's the
> difference, in mathematics, between a relation and a relationship", the
> question you raise may not arise. I declined to offer either Codd's
> or my own. I didn't offer Codd's because I don't know it. I didn't offer
> mine because I'm not a mathematician.
Well, the question was more like: a relation with attribute names in Chris Date style is more similar to a Codd's relation, to a relationship or to something in between ?
> What mathematicians who read Codd tend to overlook is that Codd was not
> writing mathematics. He was proposing a data model. The fact that his
> model maps tightly onto the mathematics of relations was part of Codd's
> intention, and one of the reasons for the durability of the model. But
> purpose wasn't to advance the frontiers of mathematics. The purpose was
> advance the frontiers of the human art and the engineering discipline of
> building and using large, shared data banks.
In the ACM 1970 paper among the operators defined I have not seen the most important one: equality even if the other operators are based on it. :-) Received on Mon Apr 10 2006 - 13:13:35 CEST