Re: Interpretation of Relations

On 2007-01-23 10:26:36 +1000, "David" <davidbl_at_iinet.net.au> said:

> Can someone outline a realistic example where we are forced to deal
> with missing information, and the approach of only storing the
> relations we know to be true using 6NF gets us into trouble?

Taking into account your reply to JOG earlier today, I can't give you a 'realistic' example. So, I apologise for not answering your question. In domains in which I work, I need to be able to handle in a consistent and domain independent manner, information which consists of

• observations (i.e. can be handled in the standard relational model)
• impossibilities (i.e. what you called 'the negation of exists' in another post)
• unknowns (i.e. the CWA would be inappropriate in the presence of the the 2nd constraint).

That's why the Darwen paper mentioned up-thread is only a start for me (or I misunderstand it). His horizontal and vertical decompositions give rise to 'special' tables that should be interpreted in a case-by-case manner. Although, to be fair to his work, his goal seems to be quite different to mine. I want to model a small number of types of 'unknown', where he seems to want to provide a framework for handling arbitrary kinds ('unsalaried', 'does not work at all', ... etc).

>
> The relational algebra allows us to deduce new facts from existing
> facts. If you have less existing facts (ie that you know to be true)
> then the only problem is that there is less you can deduce.
>
> Because of its very limited treatment of negation, RA is free from
> logical errors even when there is missing information.
>
> What more could you want?

A better treatment of negation? (Sorry, just being flippant).

Cheers,
Joe Received on Tue Jan 23 2007 - 11:13:55 CET