Re: About Entity Relation Diagram

From: Mike MacSween <mike.macsween.nospamplease_at_btinternet.com>
Date: Fri, 17 Dec 2004 14:44:09 -0000
Message-ID: <41c2f0c9$0$223$5a6aecb4@news.aaisp.net.uk>

"Gene Wirchenko" <genew_at_mail.ocis.net> wrote in message news:nmd3s0tioi7mscglip37208g0u45g5v3tt_at_4ax.com...

>>I do, however, have another question. What will the primary key of the
>>entity INTERESTS and APPEARANCE be? I read somewhere that when relating 2
>>entities with a relation, the relation doesn't have to include the primary
>>keys of the entities it relates. For example, between member and tape, the
>>relation RENTAL doesn't have to include MemverID and TapeID, only DateRent
>>and DateDue.
>
> This is not correct. You need to have the keys. What you do not
> do is show them when at the logical level. They are implied. (Think
> of how you would use that relation. You need to have the keys.)

I believe you are mistaken. They DO have to be shown at the logical, i.e. relational level. That is the logical model. Where they don't need to be shown is in an ER model.

An ER Model IS NOT the same as a relational model, it is an earlier stage of database creation which MAY lead to a relational model, but is IMPLEMENTATION INDEPENDENT. In other words an ER model could be implemented using a non relational database. Received on Fri Dec 17 2004 - 08:44:09 CST