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

From: David Cressey <>
Date: Sat, 11 Jun 2005 08:56:51 GMT
Message-ID: <nHxqe.2262$>

"Jan Hidders" <> wrote in message news:eblqe.115948$

> 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

If you told me that x is a natural number, then the value of x is not completely unknown. I know, for instance, that the value of x is NOT 3+2i or Frodo Baggins. This may seem frivolous, but it's not. Unknown values in the context of SQL NULLS are not free to be assigned any value from the universe of discourse, but only free to be assigned values from the domain of the context.

There are programming languages and data structuring schemes where variables can take on any value from the universe of discourse. The "type" of such a variable is assignable just as much as the value is. I'm not suggesting that this is desirable, but only that it's an alternative. Received on Sat Jun 11 2005 - 10:56:51 CEST

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