Re: erd to db

Hello,

Thank you for your help. In fact I am coding a software to do the mapping from Dia (ER) (http://www.lysator.liu.se/~alla/dia/) to postgreSQL (SQL statement to create my tables) and I try to find out the method or the concept behind the Dia ER plugin. I used to play with Merisse ER, and I try to find on the Internet the concept of mapping ER to relational database without a true success (the explanation depends of the writer, so I get many different versions).

Regards

Yves

--
Yves Guerin
CUSM-MUHC
Groupe Interfaces
Montreal - Canada
"Jan Hidders" <hidders_at_uia.ua.ac.be> wrote in message
news:3c9af4b6$1_at_news.uia.ac.be...


> "Yves Guerin" <yves.guerin_at_muhc.mcgill.ca> wrote in message


> news:bxpm8.7127$R8.88795_at_carnaval.risq.qc.ca...

> >

> > 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

ones


> 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

such


> thing as the primary key of a relationship other than that which is


implied


> 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

a


> table for W.


>

> -- Jan Hidders

>

>