Re: practical view updates

Peter Koch Larsen wrote:
>
> In would myself like to know the difference between a
> special value and a NULL. So far as I can tell, this distinction is subtle
> and centered around the test for equality (giving true for special values
> and null for SQL, comparisons).

        Not so subtle. A special value is a value; a NULL isn't a value, but an exception, and and undifferentiated one at that. Or to put it in other form, a special value is part of the domain, while a NULL isn't part of any but applicable to all attributes, whichever domains they are defined on.

        Where is looses all subtlety is that with undifferentiated NULLs, you have much more complex 3-valued logic -- and not even that with SQL's broken NULLs results --, and with Codd's A- and I-marks even more byzantine 4-valued logic.

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