Re: MV Keys
Date: 2 Mar 2006 06:53:32 -0800
David Cressey wrote:
> "Jon Heggland" <heggland_at_idi.ntnu.no> wrote in
> > Yes, he says "repeating groups" and "nonsimple domains" are "roughly
> > analogous", and uses a relational-valued attribute (RVA) as the (sole)
> > example of an attribute with a non-simple domain. But he presents
> > normalisation as something done for convenience.
> This is a very, very important point. If you miss the point that
> normalization was proposed as a convenience, you are going to distort the
> entire history of the adoption of the RDM both by comercial DBMS vendors and
> by database theorists. Thanks Jon.
To me this highlights the difference between 1-NF and further normalization. The RM is based on a functional mapping of column names to values. If a relation is not in 1-NF its tuples cannot be modelled functionally, and this being the core of RM, it is definitional as opposed to just mere convenience.
(And as such the RM is misnamed in my opinion. With Codd's shift from relations to relationships, it is far more a Functional Model than a Relational Model in terms of mathematics, but there you go.)
The question of MV hence distills down to the following:
What is the advantage if any of this functional mapping (one-one, where
an attribute maps to a single value) over the more general
mathematical-relational mapping (where an attribute may map to many
Is this just so the resulting constructs can be visualised as nice neat
regular tables? I am yet to hear any other reasons, but I don't
preclude their existence. Does it unduly affect the algebra somehow?
Is this just so the resulting constructs can be visualised as nice neat regular tables? I am yet to hear any other reasons, but I don't preclude their existence. Does it unduly affect the algebra somehow? Anyone?Received on Thu Mar 02 2006 - 15:53:32 CET