Re: A different definition of MINUS, part 4

From: paul c <>
Date: Thu, 01 Jan 2009 16:13:09 -0800
Message-ID: <4Zc7l.105711$sC4.22824_at_newsfe04.iad>

paul c wrote:
> <NXOR> B. Now I can try to express those clues in a different MINUS
> definition:
> R MINUS D is semantically equivalent to:
> ((R{HR1} <NXOR> R{HR2}) <AND> (<NOT> D)){HR}
> ...

Oops, just noticed a typo': the last line above (at the very least) should have read

((R{HR1} <NXOR> R{HR2}) <AND> (R{HR1} <AND> R{HR2}) <AND> (<NOT> D)){HR}

(The expression still needs work, could still have errors and maybe one of these days I'll find the right words to describe it, including the use of 'NXOR' which seems very interesting to me - I think of it as the 'equality relation'. It looks to me that when a tuple is in the NXOR result, it means that any two projections of the tuple are 'equal' as far as <AND> or natural join is concerned. I think the difference between it and the 'view' join is what McGoveran means by relative complement, as opposed to simple complement.) Received on Thu Jan 01 2009 - 18:13:09 CST

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