Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Sun, 03 Jul 2005 14:07:46 GMT
Message-ID: <SiSxe.136301$no5.7264319@phobos.telenet-ops.be>

VC wrote:
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
> news:I4Cxe.135796$Bh7.7066690_at_phobos.telenet-ops.be...
>

>>Jon Heggland wrote:

>
> [...]
>

>>>Not personally, but what more do you need than definitions of value, 
>>>domain, tuple and relation, and a minimal set of algebra operators?
>>
>>The notions of database schema, database constraints, database instances 
>>and how they are exactly related.

>
> A constraint (in the RM) is just a predicate [required to evaluate to true].

A predicate over what? Before you can define a predicate you have to define the domains it applies to.

> A relation schema is a schema name R and a set of attributes A: R(A)

You forgot to model the domains. The attributes have to be associated with a domain.

> A database schema is a pair (RR, C) where RR is a set of relation schemata
> and C is a set of constraints on RR.

You forgot to model that relations must have a unique name. And what is exaclty a "set of constraints on RR"? You didn't define that properly.

> A relation instance for R(A) is a set of tuples.

You forgot to model that the tuples have to be consistent with the header of the relation.

Jan Hidders

Received on Sun Jul 03 2005 - 09:07:46 CDT