Re: About Entity Relation Diagram

"Mark D Powell" <Mark.Powell_at_eds.com> skrev i en meddelelse news:1103294257.929421.318950_at_z14g2000cwz.googlegroups.com...
> Traditionaly in RDBMS theory the ER design is the logical design while
> the actual implementation of the tables is the physical design
>
> "Logical design in RDBMS expressed in E-R diagrams"
> http://www.cob.sjsu.edu/sadhwa_a/database%20lecture%20Bus%2093.ppt
>
> This agrees with several texts I have packed away at home, but the the
> specifics or border between logical and physical design varies by
> author.

Oooh, doesn't agree with many texts in my bookshelf, or with my understanding (and teaching) of the subject.

ER is conceptual design: What are the concepts and there associations in the business-domain).
Logical design wrt. relational database can be mapped from the ER (controlled/strengthened by normalization) and without any reference to concrete DBMS or performance issues, index etc. Physical design realises the logical design to a given DBMS. Some overlap between all 3 levels are well-known. Eg. around datatypes.

I do agree with Alfredo here.

In quite many cases, however, the conceptual design becomes modified after the logical databasedesign has revealed knowledge about the true nature of the domain. Eg. reveals normalization separate entities that might have been overlooked in the conceptual design.
Conversely, if the conceptual design is correct, it usually automatically maps to normalized tables by itself.

Therefore, the distinction between conceptual and logical design is to some extend arbitrary. Received on Fri Dec 17 2004 - 16:37:25 CET