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

Bob Badour <bbadour_at_pei.sympatico.ca> wrote:

> > 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.

Being your usual irascible self, eh Bob? The table I was describing was Flight_Schedules, there's a before, a during and an after.

That way, one can run reports on, say, Scheduled_Departure_Time against Actual_Departure_Time to check on punctuality (a stat that some airlines publish in their advertising - I know that Ryanair does).

What value would *_you_* put in for *_Actual_Departure_Time_* in such a table?

> > 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?

The relation has to exist before the flight takes off - the Scheduled time is there, the pilot's name might be there, the aircraft type might be there, but the actual time is a field which should be in the table, with a null value until such time as the Ops staff are happy that it has taken off - unless you are suggesting a separate table?

Paul...

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