Re: compound propositions
Date: Tue, 16 Mar 2010 17:12:04 GMT
>> One reason is that I still don't know how Codd's Information Principle >> applies to compound propositions, eg., " 'C1' is a customer OR 'C1' is a >> client". ...
> I suspect you're assuming an entity point of view.
> If Customer and Client are relations which represent the predicates in
> your example, and the two are exclusive, then in theory one could
> define referential constraints to exclude from each relation those
> values which exist in the other. If you have a Person relation which
> may only contain values which exist in either Customer or Client, then
> that too could be defined as a referential constraint.
Must admit I've never understood what is the important difference between a tuple and an entity, other than that some people would rather talk about entities instead of tuples. So in somebody else's vernacular maybe I am assuming an 'entity point of view', but that choice of term doesn't seem significant to me. (I'm not even sure that talking about tuples matters as far as the user or programming interface is concerned, eg., when talking only about the logical theory it matters but probably not at other times.)
If those relations were 'exclusive', I think the proposition would be something like ( 'C1' is a customer and 'C1' is not a client ) OR ( 'C1' is not a customer and 'C1' is a client ). Received on Tue Mar 16 2010 - 18:12:04 CET