On Mon, Jun 04, 2007 at 12:37:42AM +0200, PFC wrote:
> NULL usually means "unknown" or "not applicable"
Aaaargh! No, it doesn't. It means NULL. Nothing else.
If it meant unknown or not applicable or anything else, then
SELECT * FROM nulltbl a, othernulltbl b
WHERE a.nullcol = b.nullcol
would return rows where a.nullcol contained NULL and b.nullcol
contained NULL. But it doesn't, because !(NULL = NULL).
It's too bad indeed that the originators of SQL used three-value
rather than five-value logic, but this is what we have. If you
happen to want to use NULL to mean something specific in some
context, go ahead, but you shouldn't generalise that to "usually
means" anything.
A
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