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.
