This question's explanation and your answers so far.
“空港”表と“ダイヤ”表がある。F空港から出発し，K空港に到着する時刻が最も早い予約可能な便名はどれか。 空港
| 空港コード | 空港名 |
|---|---|
| A0001 | T空港 |
| A0002 | K空港 |
| A0003 | F空港 |
ダイヤ
| 便名 | 出発空港コード | 到着空港コード | 出発時刻 | 到着時刻 | 予約状況 |
|---|---|---|---|---|---|
| IPA101 | A0003 | A0002 | 12:10 | 13:05 | 満席 |
| IPA201 | A0003 | A0001 | 12:15 | 13:35 | 可能 |
| IPA301 | A0003 | A0002 | 12:45 | 13:40 | 可能 |
| IPA401 | A0002 | A0003 | 13:05 | 13:55 | 可能 |
| IPA501 | A0003 | A0002 | 13:40 | 14:35 | 可能 |
| IPA601 | A0001 | A0003 | 12:40 | 14:00 | 満席 |
IPA101 departs from airport code A0003 (airport F) and arrives at airport code A0002 (airport K), so it does meet the route condition, and its arrival time of 13:05 is the earliest among the four choices. However, its booking status is fully booked, so it does not meet the condition of being a “bookable” flight that the question requires. Looking only at the arrival times and picking the smallest one is the trap here, and this is the classic wrong answer for this question. Note that the conditions for narrowing down the rows include not only the route but also the booking status.
IPA201 has a booking status of available, and its arrival time of 13:35 is even earlier than the 13:40 of IPA301. However, its arrival airport code is A0001, which the “Airport” table shows to be airport T. It does not arrive at airport K (A0002), so it is not a flight the question is asking about in the first place. You would pick it if you compared only the times without checking the correspondence between airport names and codes, which shows how important it is to first convert names into codes with the “Airport” table and only then narrow down by the conditions.
正解 The correct answer is ウ (IPA301). This question has you trace by hand the basic operations of a relational database: linking two tables (join), narrowing down to the rows that meet the conditions (selection), and then picking the smallest value among them. The procedure has three stages. Stage 1 is converting names into codes. The “Schedule” table contains no airport names; it holds only departure airport codes and arrival airport codes. So, using the airport code as the common item, look up the “Airport” table and you find that airport F = A0003 and airport K = A0002. The flight you want is one where the departure airport code = A0003 and the arrival airport code = A0002. Stage 2 is narrowing down the rows. Three flights meet this condition: IPA101 (departs 12:10, arrives 13:05), IPA301 (departs 12:45, arrives 13:40), and IPA501 (departs 13:40, arrives 14:35). Here the question imposes one more condition, “bookable”, so a flight whose booking status is fully booked cannot be chosen. IPA101 is fully booked and is excluded, leaving the two flights IPA301 and IPA501. Stage 3 is comparing the arrival times. What the question asks for is not the flight that departs earliest but the flight with “the earliest arrival time at airport K”, so compare the arrival times with each other. IPA301 arrives at 13:40 and IPA501 arrives at 14:35, so 13:40 is earlier. Therefore the flight name that satisfies all the conditions is IPA301, and the answer is ウ. The key is not to mix up these three points: (1) look up the code table and replace airport names with airport codes; (2) narrow the rows by both the direction of departure and arrival and the booking status; (3) compare by arrival time, not by departure time.
IPA501 departs from airport code A0003 (airport F), arrives at airport code A0002 (airport K), and its booking status is also available, so it certainly is a flight that satisfies all the conditions of the question. However, its arrival time is 14:35, which is later than the 13:40 of IPA301, which also satisfies the conditions. The question asks you to pick the single flight with the earliest arrival time among the flights that satisfy the conditions, so IPA501 is not the correct answer. When more than one candidate satisfies the conditions, you must rank them by the comparison criterion specified last (here, how early the arrival time is).