Sam Mason <s...@samason.me.uk> wrote: > On Fri, Jul 24, 2009 at 09:31:13AM -0400, Tom Lane wrote: > Hope nobody minds if I go off on a somewhat pedagogic bent here! Not as long as you don't mind replies in kind. ;-) > The fact that it happens to be a NULL *value* in one case Well, according to Codd (and I tend to go with him on this) there is no such thing. NULL is a way to flag a place where a value could be stored, but is not -- because is unknown or is not applicable in that context. (He seemed to feel it was a big weakness of SQL that it didn't differentiate between these two conditions, but that's another argument.) "NULL value" is an oxymoron. >> SQL doesn't provide a test for this case that's separate from the >> test involving null-ness of individual fields. Not much we can do >> about it though. I'm not entirely sure that exposing the >> distinction would be helpful anyway ... > > I think it would The distinction between not having a tuple and having a tuple for which you don't know any applicable values seems thin. I'm not sure what that would really mean. -Kevin
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