Re: relationship in the database
Date: Fri, 20 Sep 2002 20:45:12 GMT
Having read the remainder of this thread, I see my previous comments have already been either stated or refuted several times over. My apologies for the redundancy.
"D Guntermann" <guntermann_at_hotmail.com> wrote in message
> > >
> > >Modelling of constraints an implementation decision?
> > No, I meant modelling relationships between entities with foreign keys.
> > example, suppose I have two entity types E1 and E2 and a relationship R
> > between them. Let's say R is one-to-many (from E1 to E2) and E1 has two
> > candidate keys. When translating this to the relational model I have to
> > choose which candidate key is taken as the primary key of E1 and is
> > the table for E2 as a foreign key.
> Actually, a foreign key, when implemented or modeled, can establish a
> relationship with *any* candidate key. A primary key does not have to be
> <chosen> and then always used in foreign key relationships. Thus, it
> be a better alternative to question *which* candidate key will participate
> in any foreign key relationship. This question might be asked and
> at the level of creating the conceptual model in that a user's view of the
> data might be the set of criteria that actually determines which candidate
> key might be best selected. So it might not necessarily be an
> implementation decision.
> As a sidenote, Date briefly mentions in his book, _Introduction to
> Systems, 7th Ed_, that the primary key is a traditionally accepted concept
> of the relational model, but that choosing and abiding by one is not a
> necessarily jusifiable or required condition in all situations. If I
> remember correctly, this can be found in Chapter 8 in sub-section
> primary keys and alternate keys. And if I'm not mistaken, he notes that
> Codd had previously indicated that the process of <choosing> primary keys
> was entirely out the scope of the relational model.
> Daniel Guntermann
> I would tend to see that decisions as an
> > implementation decision.
> > >I've always thought that given a particular definition of the
> > >model (say one explicitly including the concept of FKs), then the
> > >the (or at least one information equivalent and complete)
> > >logical/conceptual system catalog should simply be a direct consequence
> > >the artifacts in the definition of the model.
> > That depends upon the artifacts you choose but in general you would be
> > right. Especially for the usual suspects (key constraints, functional
> > dependencies, multi-valued dependencies, join dependencies, foreign
> > that is correct.
> > -- Jan Hidders
> > PS. Do you know what type of recursion is allowed in DB2's SQL?
Received on Fri Sep 20 2002 - 22:45:12 CEST