Re: Does Codd's view of a relational database differ from that of Date & Darwin? [M.Gittens]
Date: Fri, 10 Jun 2005 17:45:05 GMT
David Cressey wrote:
> Like some other respondents, I'm not exactly sure how Codd's ideas on
> relational databases
> and the relational data model evolved over time. I'm limiting my comments
> to the 1970 paper.
> It seems clear to me that, at that time, Codd was NOT asserting that the
> limitation to simple domains was an inherent part of the theory of the
> relational data model. Rather, he was taking the simple domain rule as a
> probable workaround to the practical obstacles to the building of the first
> relational DBMS.
I'm not sure what it means to be an "inherent part of the theory" but Codd describes non-simple domains as "undesirable" and in section 1.5 he argues in favour of flat relations. This becomes even more clear if you look at the articles that followed because, for example, the notion of "relational completeness" (and the relationship with first-order logic) doesn't make much sense if you allow nested relations.
I have the honour of being able to rub elbows with database researchers who worked on the nested relational model at the time, and they all seem to agree that this how people understood Codd at the time. Look, for example, how Roth, Korth and Silberschatz started their abstract:
"Relaxing the assumption that relations are always in First-Normal-Form (1NF) necessitates a reexamination of the fundamentals of relational database theory."
This was a paper with high visibility and AFAIK there is no subsequent paper by Codd (or Date) that reacted with the remark that no such reexamination was necessary since that assumption had never been part of the model.
- Jan Hidders