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

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Mon, 11 Sep 2006 18:48:24 GMT
Message-ID: <YbiNg.16497$9u.190748@ursa-nb00s0.nbnet.nb.ca>

Paul wrote:

>
> Bob Badour <bbadour_at_pei.sympatico.ca> wrote:
>
>
>

>>> FWIW, I try to create subtypes
>>>where there are no nulls.

>
>
>

>>That's nice. I simply don't allow null in any database I design.

>
>
>
> Why not?
>
> (Personal work experience). Aircraft. Flight Schedules.
>
> Dep time,
> Arr time,
>
> &c.
>
>
> Scheduled_Departure_Time = 'DD:MM:YYYY 10:15'
> Actual_Departure_Time = <null> untill we know different, then we put
> in a value.
>
>
> Otherwise you have to take account of the fact that these aircraft
> took off in 9999 or else in 0001 or whatever.

Or whatever. One describes the actual departure time of a flight in a relation describing actual departures and not in a relation describing aircraft.

> Null is easier. The aircraft hasn't taken off yet, we don't know when
> it will take off, or even if it will take off.

How is null easier than not inserting anything into an actual departures relation? Received on Mon Sep 11 2006 - 13:48:24 CDT