Re: Binary Relational Modeling call
Date: Fri, 16 Sep 2011 02:48:42 -0700
Message-ID: <n8ydnZ2n0dbnhu7TnZ2dnUVZ5uWdnZ2d_at_giganews.com>
Eric wrote:
> On 2011-09-15, Ivan <ivanvodisek_at_gmail.com> wrote:
>
>>"...Relations used in BRM are actually many-to-one relationships. Besides >>their regular use, they can be used to express definitions of tables >>and their columns. It also turns out that many-to-many and one-to-one >>relationships can be represented using BRM relations. Many-to-many >>relationship can be expressed using a complementary pair of BRM >>relations (experienced database users will be familiar with this >>technique). One-to-one relationships can also be expressed in similar >>way like many-to-many relationships, but with some unique constraints..."
>
> If that's a quote from somewhere you should say where.
>
> That quote, and many of your previous posts here, assume that relation
> and relationship are the same thing, and in the Relational Model they
> are not. Many people assume that the "Relational" in the Relational
> Model refers to relationships between tables, but it refers to a
It's probably best to avoid even the word "relationships". Use "referential integrity" instead.
e.g. It's a common misconception--heavily promoted by some vendors who wish to keep their customers ignorant and "locked in"--that "relation" in the name "relational model" somehow has something solely to do with referential integrity.
> Relation, defined as a set of tuples[1], which in fact corresponds to a
> table. Until you sort that out for yourself you will continue to fail to
> make sense.
>
> Eric
>
> [1] with various restrictions. Look it up!
Actually, relation is a mathematical concept with algebraic meaning. Codd's 1972 proved equivalent expressiveness between the algebra and predicate calculus.
When one really thinks about it, the use of tuples is almost as notational as the use of tables.
But certainly, while one can easily describe referential integrity constraints using relations and either the algebra or the calculus, relations themselves have little to do with referential integrity among themselves. Received on Fri Sep 16 2011 - 11:48:42 CEST