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

From: Gene Wirchenko <>
Date: Mon, 13 Jun 2005 11:54:56 -0700
Message-ID: <>

On Fri, 10 Jun 2005 18:43:22 GMT, Jan Hidders <> wrote:


>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? And it could be for very little gain? It might not be so bad if you have fixed operators, but if you have user-defined operators, it could be horrendous.


Gene Wirchenko Received on Mon Jun 13 2005 - 20:54:56 CEST

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