Re: object algebra

> > Color = {Red, green, blue, "Not Applicable", "Unknown"} [is a no, no]
>
> Closure doesn't have to do with the set, of course, but with the type -
> you're talking about operations over relations. I don't think the set above
> has a problem with closure, though. Joins, projections, etc. which reference
> an EYE_COLOR attribute can do so meaningfully and without the ambiguities of
> nulls. Given proper type definition facilities, you can do whatever you like
> with the NOT_APPLICABLE value.

Given the table:
Person EyeColor
------ --------------
Mary NOT_APPLICABLE
Bob NOT_APPLICABLE

Then, Mary.EyeColor equals Bob.EyeColor leading an AI program to conclude that Mary and Bob have the same EyeColor. This examplifies Date's conclusion: "3VL suffers from the very serious ("showstopper") problem that it does not match reality, that is results that are correct according to 3VL are sometimes incorrect in the real world". Received on Sat Feb 28 2004 - 17:31:27 CET