This question's explanation and your answers so far.
関係データベースで管理している“入館履歴”表と“建物”表から，建物名が‘東館’を条件に抽出した結果を日付の降順でソートしたとき，2番目のレコードの社員番号はどれか。 入館履歴
| 社員番号 | 建物コード | 日付 |
|---|---|---|
| S0001 | B001 | 10/12 |
| S0001 | B002 | 10/30 |
| S0002 | B002 | 10/10 |
| S0003 | B002 | 10/12 |
| S0003 | B003 | 10/29 |
| S0004 | B001 | 10/01 |
| S0004 | B002 | 10/20 |
| S0005 | B001 | 10/05 |
建物
| 建物コード | 建物名 |
|---|---|
| B001 | 中央館 |
| B002 | 東館 |
| B003 | 西館 |
For S0001, the entry date at the East Building (B002) is 10/30, which is the newest, so it is the 1st record in descending order — not the 2nd record the question asks for.
For S0002, the entry date at the East Building (B002) is 10/10, which is the oldest, so it is the last (4th) record in descending order — not the 2nd.
For S0003, the entry date at the East Building (B002) is 10/12, making it the 3rd record in descending order — off by one, so it is not the 2nd. Note that S0003 also has a West Building (B003) record dated 10/29, but that is excluded under the East Building extraction condition.
正解 First, look up the building code for the building name “East Building” in the “Building” table, which gives B002. Next, from the “Entry History” table, extract only the records whose building code is B002, giving 4 records: S0001 (10/30), S0002 (10/10), S0003 (10/12), and S0004 (10/20). Sorting them in descending order by date (newest first) gives 10/30→S0001, 10/20→S0004, 10/12→S0003, 10/10→S0002. Therefore the 2nd record is S0004 dated 10/20, and the correct answer is エ.