Re: 3 value logic. Why is SQL so special?
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Mon, 11 Sep 2006 20:06:09 GMT
Message-ID: <RkjNg.16546$9u.191406_at_ursa-nb00s0.nbnet.nb.ca>
>
>
> re:
> http://www.garlic.com/~lynn/2006q.html#22 3 value logic. Why is SQL so special?
>
> you have a row entry for a specific flight that has fields for
> scheduled and actual departures and arrival ... possibly with
> provision for computed values (difference between scheduled and
> actual).
>
> or possibly you have two different tables ... one with scheduled
> flights and another with actual flights. an entry isn't inserted in
> the table of actual flights until the flight has actually happened.
>
> the absence of a row in the actual flight table takes the place of
> null value fields (for not yet known information) in the table of
> specific flights. two tables then possibly involves referential
> integrity if there is an attempt to match an actual flight against a
> scheduled flight (in two different tables).
>
> there is also, always the possibility that you might have an
> unscheduled actual flight ... an unplanned flight for which there
> hadn't been a (pre-)scheduled departure/arrival ... but for which
> there is actual departure ... with initially a projected arrival (as
> opposed to a scheduled arrival) ... and then eventually an actual
> arrival.
Date: Mon, 11 Sep 2006 20:06:09 GMT
Message-ID: <RkjNg.16546$9u.191406_at_ursa-nb00s0.nbnet.nb.ca>
Anne & Lynn Wheeler wrote:
> Bob Badour <bbadour_at_pei.sympatico.ca> writes:
>
>>How is null easier than not inserting anything into an actual >>departures relation?
>
>
> re:
> http://www.garlic.com/~lynn/2006q.html#22 3 value logic. Why is SQL so special?
>
> you have a row entry for a specific flight that has fields for
> scheduled and actual departures and arrival ... possibly with
> provision for computed values (difference between scheduled and
> actual).
>
> or possibly you have two different tables ... one with scheduled
> flights and another with actual flights. an entry isn't inserted in
> the table of actual flights until the flight has actually happened.
>
> the absence of a row in the actual flight table takes the place of
> null value fields (for not yet known information) in the table of
> specific flights. two tables then possibly involves referential
> integrity if there is an attempt to match an actual flight against a
> scheduled flight (in two different tables).
>
> there is also, always the possibility that you might have an
> unscheduled actual flight ... an unplanned flight for which there
> hadn't been a (pre-)scheduled departure/arrival ... but for which
> there is actual departure ... with initially a projected arrival (as
> opposed to a scheduled arrival) ... and then eventually an actual
> arrival.
And your point would be? What is the theoretical foundation or even the ad hoc design criterion to choose among the options you present? Received on Mon Sep 11 2006 - 22:06:09 CEST