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Re: definitions

From: David Cressey <David_at_DCressey.com>
Date: 2000/04/30
Message-ID: <uKZO4.128$s4.14221@petpeeve.ziplink.net>#1/1

What follows is in my experience.

The E-R model is a conceptual data model. It is independent of which technology you use to implement the solution. The conceptual data model is primarily useful for capturing in one place the result of analyzing the data requirements of an entire application, department, or enterprise. It's an analysis model.

The E-R model, as you state consists of Entities and Relationships. There are also attributes, which are characteristics of the entities or relationships. Attributes are described by data.

The relational model is a design model. It is, of course, most useful when the final implementation is going to be on a relational DBMS. It's also relevant, although less so, when the implementation is to be on some system other than a relational DBMS.

The relational model is composed of schemas, tables, columns and domains. Most writers add indexes to this list, although strictly speaking the indexes are about providing access to data, and not about modeling the data.

There is a rough correlation between tables and entities, and between attributes and columns. Relationships are generally expressed by the connection between foreign keys and corresponding primary keys. Many to many relationships require tables of their own, with two or more foreign keys providing the connection to the relevant entities.

There are CASE tools that can manage conceptual data models using E-R modeling and
logical or physical data models using Relational modeling (for the logical part). They can
even automatically generate a logical model from a conceptual model, or vice versa.

Hope this helps.

Dave Cressey
david_at_dcressey.com

Ken wrote in message ...
>I have a few questions about database models...
>
>For a somewhat formal definition of the E-R data model, I know that an
>entity-relationship model consists of a set of objects entities and a set
 of
>relationships among those objects. I can't seem to find a definition for
>the relational data model
Received on Sun Apr 30 2000 - 00:00:00 CDT

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