Re: Foreign key problem

From: paul c <>
Date: Fri, 16 Jun 2006 16:39:49 GMT
Message-ID: <p9Bkg.46469$IK3.24667_at_pd7tw1no>

x wrote:
> "paul c" <> wrote in message
> news:Szfkg.33274$IK3.15126_at_pd7tw1no...

>> If Codd used the word glue I imagine it might have been as a synonym for
>> coherence, which I could buy if I knew his context.

> Here is the context you asked for:
> Chapter 3: Domains as Extended Data Types, page 44.
> Section 3.2 Nine Practical Reasons for Supporting Domains:
> "_First_, full support of the domain concept is the single most important
> concept in determining whether a given relational database is integrated.
> Consider the consequences of alleging that a relational database viewed as a
> collection CD of domains and a collection CR of relations could be split
> into two databases, without any loss of information or of retrieval
> capability."
> "This first reason for supporting domains can be concisely stated as
> follows: _domains are the glue that holds a relational database together_.
> Notice that I said _domains_, not primary keys and foreign keys. The concept
> of keys in the relational model provides an important additional and
> specialized kind of glue."
> From what I've read so far, the importance of domains is emphasized in
> several chapters.
> ...

Thanks very much for typing that in. It does look to me as if Codd had coherence in mind because he talks of holding a relational database together even though his example may be too deep for me. The test of splitting the database eludes me because I don't see how domains and relations could be separated. I thought of a relation as being a set of sets of mappings between individual values of domains, meaning to me that loosely, relations result from "gluing" subsets of domains together so I didn't see how one could 'subtract' domains and still be able to 'see' a relation.

p Received on Fri Jun 16 2006 - 18:39:49 CEST

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