Re: Foreign key problem
Date: Fri, 16 Jun 2006 16:39:49 GMT
> "paul c" <toledobythesea_at_oohay.ac> wrote in message
>> 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
> "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