First - thank you. I want to again thank this list, which in general is the most
tolerant to people new to database use and theory in general, and me in
particular. I mostly got your point from an earlier answer. Hopeful your example
quashes my ignorance, relative to NULL at least.
I think I understand the manual examples with the help of the answers I got. I
was not getting it on my own.
On Tue, 20 Sep 2005, Joerg Bruehe wrote:
> Just some explicit addition:
> doug@stripped wrote:
> > [[...]]
> > So in the following query:
> > select * from new_payments where closed<>1;
> > it is desired that null=1. DeMorgan's law takes a vacation here.
> You use two-valued logic here, where statements are either "true" or
> "false". (DeMorgan's law applies to two-valued logic only.)
> When NULL values are not excluded, SQL uses a three-valued logic, where
> a stament may also be "unknown". Comparing NULL to any value (including
> a comparison of NULL and NULL) always results in "unknown".
> This also the reason that the SQL syntax does not allow
> ... WHERE value = NULL
> but requires that you write
> ... WHERE value IS NULL
> It has also been said that NULL is no value but a state - maybe that
> helps in understanding.
> Joerg Bruehe, Senior Production Engineer
> MySQL AB, www.mysql.com