On Jul 23, 2009, at 6:06 PM, Jeff Davis wrote:
However, in step 2, you transformed:
x IS NOT NULL => NOT x IS NULL
But in SQL that is not a tautology!
No, that's not the problem I see -- that solved the problem in my
particular code. The problem I see is that, given that the standard
says (according to Tom) that if any value is NULL then the record is
NULL, then I would expect this to return false:
SELECT ROW(1, NULL) IS DISTINCT FROM ROW(2, NULL);
But it returns true.
I don't think it's wise to assume SQL is consistent. I think it is
possible to create a consistent 3VL system, but you have to give up
some
other very important property. I can't remember the details at the
moment, but there's an interesting proof in "Logic and Databases" by
C.J. Date.
If the standard says that, in the case of records, two NULLs are
distinct, then fine. Completely bizarre, but fine. But I suspect that
such is not the case.
Best,
David
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