loans 表里的 days 是借了几天；还没还的书，days 是空的（NULL）。算出 days 的平均数（空的不算），用 ROUND(…, 1) 保留 1 位小数。
| id | title | author | genre | pages | year |
|---|---|---|---|---|---|
| 1 | The Night Garden | Mia Lane | story | 210 | 2015 |
| 2 | Stars for Kids | Tom Hart | science | 96 | 2020 |
| 3 | Lost at Sea | Mia Lane | story | 340 | 2011 |
| 4 | Easy Bread | Rosa Bell | food | 128 | 2019 |
| 5 | The Bug Book | Tom Hart | science | 64 | 2022 |
| 6 | Soup Days | Rosa Bell | food | 150 | 2008 |
| 7 | The Red Kite | Ann Cole | story | 180 | 2021 |
| 8 | Rocks and Rivers | Ann Cole | science | 220 | 2017 |
| id | book_id | reader | days |
|---|---|---|---|
| 1 | 1 | Leo | 14 |
| 2 | 2 | Nora | 7 |
| 3 | 1 | Sam | NULL |
| 4 | 4 | Leo | 21 |
| 5 | 5 | Nora | NULL |
| 6 | 3 | Ivy | 10 |
| 7 | 2 | Sam | 5 |
| 8 | 7 | Ivy | 12 |
| id | title | author | genre | pages | year |
|---|---|---|---|---|---|
| 1 | The Night Garden | Mia Lane | story | 210 | 2015 |
| 2 | Stars for Kids | Tom Hart | science | 96 | 2020 |
| 3 | Lost at Sea | Mia Lane | story | 340 | 2011 |
| 4 | Easy Bread | Rosa Bell | food | 128 | 2019 |
| 5 | The Bug Book | Tom Hart | science | 64 | 2022 |
| 6 | Soup Days | Rosa Bell | food | 150 | 2008 |
| 7 | The Red Kite | Ann Cole | story | 180 | 2021 |
| 8 | Rocks and Rivers | Ann Cole | science | 220 | 2017 |
| id | book_id | reader | days |
|---|---|---|---|
| 1 | 1 | Leo | 14 |
| 2 | 2 | Nora | 7 |
| 3 | 1 | Sam | NULL |
| 4 | 4 | Leo | 21 |
| 5 | 5 | Nora | NULL |
| 6 | 3 | Ivy | 10 |
| 7 | 2 | Sam | 5 |
| 8 | 7 | Ivy | 12 |
-- 在这里写：用 ROUND(AVG(days), 1) 算平均天数 SELECT days FROM loans;
写好了先点「运行」看看结果，没问题再点「提交」。