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Typical errors at solution of exercises. Exercise 40
S. Moiseenko
For each company, find time the company's planes have spent during accomplished flights.
Result set: company name, time in minutes.
The issue with this exercise may be illustrated by a message of one of our participants:
SELECT Trip.time_out, Trip.time_in
FROM Trip
WHERE Trip.id_comp=2
time_out time_in
-----------------------------------------------
1900-01-01 09:35:00.000 1900-01-01 11:23:00.000
1900-01-01 17:55:00.000 1900-01-01 20:01:00.000
It turns out that the company Aeroflot has made two flights, the first one has flight
time of 1 hour 48 minutes, the second one - 2 hour 6 minutes, The total duration
of flights turns out (108 + 126) = 234 minutes, but not is 216 minutes (as it is
specified in "correct result ")
Misunderstanding is caused by insufficient studying of the description and the scheme
of database. Trip table represents the schedule of flights, which are carried out
daily.
The information on flights of passengers contains in Pass_in_trip table. Let's look
what flights of the company with id_comp=2 have been carried out:
select pt.trip_no,date,time_out,time_in
from pass_in_trip pt
join
(select trip_no,time_out,time_in from trip where id_comp=2)
t
on t.trip_no=pt.trip_no
group by pt.trip_no,date,time_out,time_in
Here is the result of the query above:
trip_no date
time_out time_in
--------------------------------------------------------------------------------
1145 2003-04-05 00:00:00.000 1900-01-01 09:35:00.000 1900-01-01 11:23:00.000
1145 2003-04-25 00:00:00.000 1900-01-01 09:35:00.000 1900-01-01 11:23:00.000
So, the first flight has been carried out twice, but the second one did not be carried
out at all, i.e. 108*2 = 216.
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