Re: At what ANSI/SPARC level are you, when creating new... totally lost
Date: Sun, 15 Jul 2007 08:45:22 -0000
On 14 jul, 17:20, kaja_love..._at_yahoo.com wrote:
> This is really confusing
> >>Thus, by creating a new database are you creating logical model or
> >>conceptual model?
> >The external model and the conceptual model are both logical models.
> But the quote below suggest that logical model is part of internal
> schema and thus conceptual model can't also be logical model?!
> >>An internal schema is an organization of data according to the technology [...snip...]
No wonder you are confused. In his article David Hay mixes ANSI/SPARC terminology with general data modelling terminology and pretends they are the same. This is not the case, especially terms such as "logical model", "internal model" and "conceptual model" have sometimes subtly different meanings. As a consequence he actually misrepresents the exact meaning of the ANSI/SPARC terminology. The ANSI/SPARC layers were meant to describe the internal archtitecture of a certain DBMS, and therefore, by David Hay's definition of the term, are actually *all* in the internal layer. What he calls the conceptual schema and external schema are completely outside the range of what the ANSI/ SPARC architecture attempts to describe.
> Since I only know ( a little ) about relational database, I'm going to
> ask the following question in the context of relational database:
> The way I understand the above paragraph is that logical schema
> ( which the article claims is a part of internal schema ) deals with
> tables, while conceptual level deals with objects ( entities ) and
> thus knows nothing about tables and keys. Uh, what am I missing here?!
As the terms are usually used in a data modelling context this is largely correct. Btw., entities, of course, also have keys.
- Jan Hidders