Re: Objects and Relations

On Feb 13, 4:49 am, "JOG" <j..._at_cs.nott.ac.uk> wrote:

> RM does not take this view. It is not concerned with 'entities', but
> facts - propositions composed of roles and values.

The above two sentence (it seems to me) have the unusual ideas and its logic is not consequential enough.
As these are the basic things in RM I have the following remarks:

1)

E. Codd in his paper RM/T has the chapters about the entities:
4. Designation of Entities
5. Entity Types
6. Classification of Entities
7. Entities and their immediate Properties.
.....

The paper was written 10 years after his first paper about RM. He had a lot of time to think about RM, now in better scientific and technical environment. In introduction of RM/T and regarding semantic data modeling E. Codd wrote:
"The goal is nevertheless an extremely important one because even small successes can bring understanding and order into the field of database design."
 Now regarding your ideas it seems that: RM is not concerned with 'entities' but the author of RM is concerned with 'entities'

2)
RM works with the facts but this is not specific only for RM. Every DB model works with the facts. In fact I can't imagine that some DB model - models non-facts.

3)
I have a feeling that you don't distinguish completely Conceptual Model from Logical Model. In this sense you can't substitute the concept of the entities with the logic of the propositions.

4)
The story about the propositions can lead you back toward the design style of "programming-files-based" application. Well known example from the books about the propositions in RM exists in fact in the Cobol applications Example:
Let a Cobol application has the following "logical record" for Supplier with "fields" : SNO, SNAME, SSTATUS, and SCITY. This heading for the file represents a certain predicate which looks like: Supplier SNO is under contract, is named SNAME, has status SSTATUS, and is located in city SCITY.
So regarding "logical" description of data structure the relational RM use the similar ideas as old Cobol applications. Of course there are differences as well as the advantages and the disadvantages. The relational applications use the relations, they are DB, etc.

5) DB Design is the most important step in the construction of DB. It is very complex - much more then Predicate Calculus.

Vladimir Odrljin Received on Tue Feb 27 2007 - 17:25:24 CET