Re: DB design using designer/2000???

You are very confused about the concepts involved here.

1. An intersect table is only necessary when a Many to Many relationship exists.
2. A One to Many always maps to the resulting Many side having a Foreign Key to the table implementing the One side. Foreign keys may be either optional or mandatory; in the case of an optional foreign key, the foreign key column allows NULL, and there is no corresponding parent record in the referenced table. This is *fully* in accord with relational database theory. See any book on relational theory.

A couple of other points...

In the ERD, the lines between the entitys are called "relationships", not "relations". "Relations" are another term for the tables themselves (from the orginal mathematical works).

The (M:M) notation is usually used to refer to the maximum cardinality of the relationships, usually expressed as (1:1), (1:M), (M:M), but does not express the optionality of the relationships, i.e. *not* (O:O).

Finally, good ER modelling practices dictate that you should have eliminated all of Many to Many relationships before proceeding to the design stage. Many to Many relationships are almost always hiding additional important information that can only be discovered by resolving them to well named intersect entity and then examining the resulting entity for missing attributes, relationships, etc.

Oracle Owner <oracle_at_engr.csulb.edu> wrote:

>This is a database design question.
> The question is about the Designer/2000, when I use the
> "Database design wizard" to convert the entity relationship diagram
>that I designed, it treat the entity relatioship "many to 1(O:O)" is
>same as "many to 1(M:M)", my question is if there is no intersection
>table between the entities of the many to 1(O:O) relationship, this means
>that there will be the null valuse exist in the foreign key field of the
>"many" side entity's mapping table, how does the "referential integrity"
>constraints be enforced in this case, and have the null value in the
>relation is against the principle of databse design. why the Oracle designs the
>CASE tool in this way??? What is the concept behind the design.
> If I really need a intersection table between the many to 1(O:O)
>entities, is there a way I can create one after I perform the "database
>design wizard" to create the mapping table base my ER diagram.
>
> Thanks in advantance
> Katherine
>
>

-JJ

 Jeffrey Jacobs & Associates
 Oracle Designer/2000 Consulting & Training  951-2 Old Country Road, Suite 119, Belmont, CA 94002  Voice: 415-571-7092, FAX: 800-665-1379
 Email: jmjacobs_at_ix.netcom.com Received on Tue Jun 17 1997 - 00:00:00 CEST