Re: ER diagram question

jonck_at_vanderkogel.net (Jonck van der Kogel) wrote:
>> It's only a better style if it's correct. Now look at a Project Communication
>> and a Customer Communication to see if there are any differences in attributes.
>> One possible solution is to create a super/sub type entity on the Communication
>> entity. One subtype called Customer Communication would have a relationship
>> to Customer, and the other subtype, Project Communication would have a
relationship
>> to Project.
>
>Ok, I had something like that doodled here on my sketchpad :-) But
>it's necessary for a communication to be linked to a project as well
>as a customer. (Bernard: thanks for your suggestion, I agree that in
>some cases that's possible, but in my case the projects are of thus
>size, that a customer will not have concurrent projects running, thus
>they will always only communicate about 1 specific project)
>So how would I translate the super/sub solution to real tables? As I
>currently understand it, no matter how I do it the table will have to
>have a customer_id and a project_id field, thus creating circularity
>in my diagram, correct? Or am I missing something here?
>Thanks again for your help, Jonck
Review primary keys and identifying relationships. No need for subtypes if Communication is dependent on Project. Received on Wed Jan 21 2004 - 18:31:13 CET