Hi Dissip, Welcome to JavaRanch. I'll try and explain a way that this might work - you will have to decide for yourself whether this fits or not. In the airline industry, they quite often change the type of plane for a given flight depending on the number of bookings. So even if you caught the same flight number every day between the same cities at the same time, you might get a different type of aircraft. For example a 747 holds roughly 400 passengers (it varies by 747 model number), whereas a DC-10 holds roughly 300 people - either might be used for the same route, depending on the number of passengers booked for a certain flight on a certain day. Furthermore the seating types themselves can change on any given day (although this doesn't normally apply to the low cost airlines). So today's flight might have 9 first class seats, 30 business class seats, and 335 econonmy class seats. But tomorrow's flight on the same physical aircraft may have 6 first class seats, 20 business class seats, and 358 economy class seats - all dependant on what sort of bookings the airline has already taken and what their expected usage of the flights will be. So, in a sense, each individual flight can be considered to have a unique aircraft. Does this help? Regards, Andrew [ March 12, 2004: Message edited by: Andrew Monkhouse ]
Thank you very much, Andrew. And what's your opinion about the 1-1 relationship between the segment and the flight? As I think a segment is a part of the itinerary reservation. And a flight is a trip of an airplane (equipment) from one city to another at specific date and time. So it is reasonable that every segment correspond to 1 flight. But how could a flight just correspond just 1 segment (reservation). (Although I tried to interpret the segment and flight as some other things, such as segment: the route between 2 cities, flight a series of trips of an airplane from one city to another, it seems there's always a contradiction in the multiplicity of the relationships.) Thank you again and looking forward for your reply.