Re: Newbie help with ER diagrams
Date: Sat, 7 Dec 2002 03:47:26 -0800
Message-ID: <ZdkI9.2527$K5.1340_at_fe01>
Hi Jan Hidders. Thanks for the info. It seems that ER diagrams don't really have a "standard" then, do they?
me
"Jan Hidders" <hidders_at_hcoss.uia.ac.be> wrote in message
news:3def1a82$1_at_news.uia.ac.be...
> In article <LdEH9.25136$EY.4943_at_fe01>, me <toenails777_at_hotmail.com> wrote:
> >Hello all. I was wondering if anyone could help me with an ER diagram
I'm
> >trying to build, but I don't see how to do it. I can see how to do it in
a
> >programming language say C++, but not with ER diagrams.
> >
> >Suppose I have three entities: A, B, and C. B "is a" A, and C "is a A"
> >also, but B also "has a" C. In programming, I'd have:
> >
> >class A
> >{
> >};
> >
> >class C: public A
> >{
> >};
> >
> >class B: public A
> >{
> > C myCClass;
> >};
> >
> >In an ER diagram, I guess I want to express aggregation. So I have:
> >
> > -------------
> > | A |
> > -------------
> > | |
> > | |
> > ------ ------
> > \ is a / \ is a /
> > \ / \ /
> > \ / \ /
> > | |
> > | |
> > ------ ------
> > | B | | C |
> > ------ ------
> >
> >How would I express aggregation between B and C?
>
> You add a relationship between B and C with a name like 'contains'. The
> cardinalities for your C++ example are something like
> - every B contains at least one and at moste one C (i.e. exactly one C)
> - every C is contained in at most one B
> but this may depend upon what you are modelling exactly. Some ER
techniques
> have a special notation cq. name for such aggregation relationships, and
> some don't, so that depends on what ER technique you are using exactly.
>
> -- Jan Hidders
> > > >Received on Sat Dec 07 2002 - 12:47:26 CET