Re: 3 value logic. Why is SQL so special?

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Tue, 12 Sep 2006 19:26:44 GMT
Message-ID: <URDNg.19082$9u.199830@ursa-nb00s0.nbnet.nb.ca>

Paul wrote:

> "JOG" <jog_at_cs.nott.ac.uk> wrote:
>

>>>>That's nice. I simply don't allow null in any database I design.
>>>
>>>Why not?
>>
>>you're kidding right?

>
> No.
>
>

>>>Scheduled_Departure_Time = 'DD:MM:YYYY 10:15'
>>>Actual_Departure_Time = <null> untill we know different, then we put
>>>in a value.

>

>>So an aircraft might not take off and yet it has a departure time
>>attribute? That makes absolutely no sense at all.

>
> I see that you are from Nottigham university - I would have thought
> that a reasonable grasp of the English language was necessary to
> either take or give courses there.
>
> What part of "Scheduled" did you not understand?
>
> I can *_schedule_* a flight for Dublin airport for 6 months time - the
> damn airport might have been dirty bombed by Osama and his pals in the
> meantime, meaning that the aircraft will never take off from there.
>
> I don't know how familiar you are with aviation, but *_actual_*
> departure time is important - pilots fill out VR's (Voyage Reports)
> which are legally binding documents which have to have the actual
> departure time (apart from pay issues for staff).
>
> Now, if the field "Actual_Departure_Time" is not null, then it has to
> be something - yes?

Not if it doesn't yet exist.

> That "something" is normally some arbitrary value - i.e. 01/01/1970 or
> whatever - the point is any application programmer has to take that
> system defined arbitrary value into account when writing code against
> the system.

Since your assumption that it has to be something is false, everything you predicate on the assumption is unsound.

> *_WHEN_* the value used is <null>, then the programmer doesn't need
> any special knowledge of the system - i.e. "By the way, the date for
> planes which haven't taken off yet is xxx/yyy/zzzz" - just a simple,
> "Oh, that's null until the VR is filed".
>
> What could be simpler or more logical?

Logic. Received on Tue Sep 12 2006 - 14:26:44 CDT