Re: Does Codd's view of a relational database differ from that of Date & Darwin? [M.Gittens]
Date: Sat, 11 Jun 2005 00:47:03 GMT
Message-ID: <bwqqe.116145$zd5.6141040_at_phobos.telenet-ops.be>
Marshall Spight 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. :-)
>
> Hmmm. I will agree that for any unknown value of x, you can say
> things about that are also true for all x.
>
> Hmmm. That makes it seem like UNKNOWN is really a universally
> quantified logic variable.
Not precisely, because "x > 5" would then be "false" but under the "unknown value" interpretation the answer should be "maybe, maybe not".
> That sort of makes it seem like arguing for UNKNOWN values
> is like arguing for universally quantified logic variables, which in
> the
> absence of unification are neither interesting nor useful. Or am I
> wrong?
It's interesting if only because it is the formal definition of what the "missing value" interpretation actually means. However, it is an unattainable ideal, although unification is certainly not the only nor always the best way to deal with universal quantification, and the situation is certainly not as hopeless as you make it out to be. Nevertheless, if you only want to approximate the ideal you should still know and understand it.
- Jan Hidders