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

Marshall Spight wrote:
> My comment on this thread would be that the problems here
> arise not from missing information per se, nor from the empty
> set, special values, or whatever. The problems arise from
> 3VL. Once you introduce an unknown value, you can't
> say anything about anything. Is x = UNKNOWN? It might
> be, it might not; you can't tell. What about the sum of a set
> of numbers and an UNKNOWN? Nothing can be said about it.

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. :-)

• Jan Hidders