Re: The Theoretical Foundations of the Relational Model

In article <3d19da2d.3083163_at_news.verizon.net>, JRStern <JXSternChangeX2R_at_gte.net> wrote:
>
> So, if a product is going to support the three-layer model, they would
> have two kinds of views, internal and external, and one would allow
> sort and the other would not?

Well, I would say that views are always part of (the mapping from the conceptual level to) the external level so there would be only one sort and it would allow ordering. But this placing of views in the external level is not uncontroversial.

>If you have a reference you think makes the three-level model seem
>sensible, please suggest it. I'll take another look next time I'm at
>the bookstore.

Try Date's seventh edition, section 2.2 "The three levels of the architecture" (but earlier editions also have it). Actually most introductory works on databases will have something on it. If you are a member of ACM you could also try the original report:

 http://www.informatik.uni-trier.de/~ley/db/journals/sigmod/BurnsFJKMRRRT86.html

> The three-level model *seems* like a good idea, and maybe it can be
> salvaged, but the only encounters with it, or its advocates, I've ever
> had have been massively unconvincing.

The reason that I brought it up is because it helps to explain what it is that the RM exactly claims and how this claim is supported. If you understand that then you also may understand when the RM is not such a good idea.

• Jan Hidders