Re: On formal HAS-A definition

From: Nilone <reaanb_at_gmail.com>
Date: Sat, 8 May 2010 11:14:18 -0700 (PDT)
Message-ID: <e05d8e24-950c-4561-b62c-19380a60d216_at_p5g2000pri.googlegroups.com>


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

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