Re: erd to db

"Yves Guerin" <yves.guerin_at_muhc.mcgill.ca> wrote in message news:nDGm8.10802$R8.275980_at_carnaval.risq.qc.ca...
>
> 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.

Are you sure it exists? Usually drawing programs try to incorporate as much concepts as possible to accomodate as many ER techniques as possible. If you give a list of the concepts it supports then perhaps we can help you.

> 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).

That is only going to get worse if you are going to ask opinions here. :-) The problem is that this mapping involves certain implementation decisions and may depend upon details that are not in the diagram. In your case I would make the mapping as straightforward as possible so that the relationship between the ER model and the tables is as simple as possible. So I would choose to map every relationship (type) and every entity (type) to one table. That's easier for the user to understand, and easier for you to implement. :-)

You asked for references. I have taught from Date, Elmasri/Navathe and Ullman/Widom ("A First Course in Database Systems") and liked all of those with a light preference for the last one. The basics are pretty straightfoward and the same everywhere:
- entity type -> table
- M:N relationship -> table

But then things get hairy:
- 1:N relationship -> foreign key on N-side or separate table? What if the relationship has attributes?
- ISA relationship -> ... several options here ... - weak entity types -> table for the weak relationship or foreign key in table for weak entity?

et cetera ...

Kind regards,

• Jan Hidders