Re: On formal HAS-A definition

On May 8, 2:12 pm, r..._at_raampje.lan (Reinier Post) wrote:
> Nilone  wrote:
> >It looks to me like tuples are being equated with entities.
>
> When justifying the definition of IS-A in the E/R-model: yes.

What do you mean? I'm thinking:

• E/R model entities and attributes are elements of domains
• E/R model relationships are relations over domains
• Tuples are the elements of relations over domains of entities
• IS-A is an isomorphism between domains of entities
• HAS-A is a monomorphism between domains of entities

So when it looks like an entities are equated with tuples, I wonder why. I believe it's a mistake to do so, but perhaps I'm missing something.

> >A relation is not a domain.
>
> THat is irrelevant for the IS-A I'm discussing.

My apologies for jumping to conclusions. I would like to understand the IS-A you're discussing. I'll continue reading. Received on Sat May 08 2010 - 20:14:18 CEST