Re: erd to db

From: Yves Guerin <>
Date: Fri, 22 Mar 2002 13:57:40 GMT
Message-ID: <oLGm8.10804$>


Do you have any poionter to a document or a web page or a book about doing a corect ERD (see the message (post) before)?



Yves Guerin
Groupe Interfaces
Montreal - Canada
"Jan Hidders" <> wrote in message

> "Yves Guerin" <> wrote in message
> news:bxpm8.7127$
> >
> > I have question about to map a erd to a ralational database:
> >
> > I have the following entities: E and W in binary ERD
> > E: owner of W
> > W: weak
> > E: primary key is e1
> > W: primary key is w1
> > The relationship between E and W is R and R has a primary key: r1
> I'm not sure what type of ERDs you are talking about, but in the usual
> your problem statement doesn't make sense. First, if W has a primary key
> then it is by definition not a weak entity, and second, there can be no
> thing as the primary key of a relationship other than that which is
> by the involved entity types (if there would be then it is not a
> relationship but an entity type).
> So it should have been:
> An entity type E with primary key e.
> A weak entity type W with a partial key w.
> A weak relationship R between E and W.
> The usual way to implement this is to introduce a table for E with primary
> key e, and a table for W with primary key e + w. Note that you don't need
> table for W.
> -- Jan Hidders
Received on Fri Mar 22 2002 - 14:57:40 CET

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