homework - trying to calculate multiple flight stops between two cities? - sql

Question

This is for a database class, using Oracle DB 11g
Tables:
• Flight (flt_no, from_city, to_city, flt_distance,
flt_departs, flt_arrives, flt_price)
• Aircraft (craft_id, craft_name, cruising_range)
• Employee (emp_id, emp_name, emp_salary)
• Certified (emp_id, craft_id)
QUERY: A customer wants to travel from Madison to New York with no more than two changes of flight. List the choice of departures from Madison if the customer wants to arrive at New York by 6pm.
ANY Help is appreciate for this. I really don't even know where to start on this.
EDIT
This is what I've come up with so far. Please let me know if I'm on the right track at least please.
SELECT F.flt_no
FROM Flight F
WHERE F.from_city = 'Madison'
AND F.to_city = 'New York'
AND DATEPART(hh, F.flt_arrives) <= 18
UNION
SELECT F.flt_no
FROM Flight F
WHERE (F.from_city = 'Madison'
AND F.to_city IN (SELECT from_city
FROM Flight F
WHERE F.to_city = 'New York')
)
OR
(F.to_city = 'New York'
AND F.from_city IN (SELECT to_city
FROM Flight F
WHERE F.from_city = 'Madison')
AND DATEPART(hh, F.flt_arrives) <= 18
)

Answer 1

Think of the three cases:
A direct flight.
flight with 1 stop.
a flight with two stops.
Now start stepping through the cases.
For 1. Look for flight that start and stop, you know where and you know when by, right?
For 2. Think about those two flights. Where will they start and end, what will be in common with the two flight. What's the time criteria for each flight?
For 3. Think about those 3 segments and the characteristics of each, including dependencies
Then you finally want the union of all these if all choice are to be shown.
This function: select to_char(DATE,'HH24') may help.

Answer 2

As #Michael Durrant has given the answer I just want to contribute the solution in SQL format.Hope this will help.Please check the Not Equal to sign.
#Direct flight
select f.no
from flights f
where f.arrives=18:00 and (f.from='madison' and f.to='NewYork')
union
# Cases for one connecting flight
select f2.no
from flights f1, flights f2
where f2.arrives=6pm and
f1.from='Madison' and f1.to<>'New York'
and f1.to=f2.from and f2.to='New York'
and f2.depart> f1.arrives
and f2.arrives<'18:00'
union
#Cases involving two connecting flights
select f3.no
from flights f1,flights f2, flights f3
where f1.from='Madison'
and f1.to=f2.from
f1.to<>'New York'
f2.to=f3.from
f2.to<>'New York'
and f3.to='New York'
and f2.depart> f1.arrives
and f3.depart> f2.arrives
and f3.arrives<'18:00'