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$no5.7264319_at_phobos.telenet-ops.be>
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> [...]
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> A constraint (in the RM) is just a predicate [required to evaluate to true].
Date: Sun, 03 Jul 2005 14:07:46 GMT
Message-ID: <SiSxe.136301$no5.7264319_at_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 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.
> 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