Re: Does Codd's view of a relational database differ from that of Date & Darwin? [M.Gittens]

From: Jan Hidders <>
Date: Tue, 14 Jun 2005 06:58:54 GMT
Message-ID: <Oevre.120272$>

Gene Wirchenko wrote:
> On Fri, 10 Jun 2005 18:43:22 GMT, Jan Hidders
> <> wrote:
> [snip]

>>Sometimes you *can* say something, even though some of the variables are 
>>unknown. Suppose I told you x is a natural number, could you then tell 
>>me whether the following statements are true:
>>  (x > 5) or (x <= 5)
>>  (x - x) = 0
>>I bet you could. :-)

> Your first example is equivalent to
> (x>5) or !(x>5)
> and is a case of excluded middle, but establishing that equivalency
> requires considering the internals of > and <=. That means that you
> can not use a truth table.
> Can you imagine the difficulty of figuring out all such possible
> interations of operators?

It's undecidable, even if you only consider equality as an operator, which arguably doesn't have any internals. I already mentioned that earlier on in the thread. So I'm not sure what exactly your point is here.

  • Jan Hidders
Received on Tue Jun 14 2005 - 08:58:54 CEST

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