Re: Interpretation of Relations

David wrote:
> 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?

Hi David. My example was in 6NF, and I believe it generates a contradiction when missing information cropped up. However, if you think it is missing a trick somewhere then isolating that would obviously be very useful.

>
> 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.

Splitting off nullable attributes at least accords to relational theory, so I employ 6NF in my schema, especially now I have decent OR tools. However I'm sure you know that even with 6NF, whence I can omit the nulls that might have occured in the denomarlized form, I am probably not stating the full extent of the information I had (whether info is absent because it is missing or not). CWA infers that if I do not state anything, then the projected attribute was inapplicable, when it was perhaps applicable and missing, or perhaps its applicability was unknown altogether. Obviously this information has an impact with queries aggregating over the relevant attributes. Darwen suggests a solution that includes extra relations to denote these facts (as does the transrelational model I believe, which I suspect tries to correlate all these facts to facilitate less convoluted queries), but I currently find something about this mechanism logically unsatisfying - without being able to explain exactly why yet.

>
> 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?
Received on Tue Jan 23 2007 - 02:29:05 CET