Re: Max date

On Wed, Sep 23, 2015 at 4:58 PM, Charles Wilt <charles.wilt@xxxxxxxxx> wrote:

On Wed, Sep 23, 2015 at 2:13 PM, John Yeung <gallium.arsenide@xxxxxxxxx>
wrote:

Let's say I have a nullable variable call MYVAR. And let's say that
its null indicator is currently on. So in SQL, MYVAR is simply equal
to NULL. [...]

In SQL, MYVAR > 2 would be False, since any comparison with NULL is
False.

No, no it wouldn't and no, no it isn't...

In SQL,
when MYVAR is NULL
then
MYVAR > 2 is unknown; neither true or false.

Fine. I made the all-too-common verbal fumble of not pointing out
where the three-valued logic collapses to two-valued logic for
purposes of program flow. (That is, a truth value of "Unknown" is just
as good as "False" when you have a piece of code that is conditionally
executed, such as an IF block. Similarly, in SQL, a record has to
either be in the result set or not in the result set; it's not like
there's a "well, maybe it's in the result set, but we don't know".)

I'm sure you realize this, but it just goes to show how easy it is to screw
up 3 valued logic.

I can agree with that. I will say that it's both easy to screw up the
logic itself, and also easy to screw up talking about the logic.

John Y.