Re: Does Codd's view of a relational database differ from that of Date & Darwin? [M.Gittens]
Date: Tue, 14 Jun 2005 06:58:54 GMT
Gene Wirchenko wrote:
> On Fri, 10 Jun 2005 18:43:22 GMT, Jan Hidders
> <jan.hidders_at_REMOVETHIS.pandora.be> 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. :-)> and is a case of excluded middle, but establishing that equivalency
> Your first example is equivalent to
> (x>5) or !(x>5)
> 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