SQL发现每个地区每个学生平均送出kg。

I have three tables

我有三个表

school
| id      | area       
|       1 |      area1
|       2 |      area1 
|       3 |      area1 
|       4 |      area2 

class
| id      | school     | students  |
|       1 |          1 |     2     |
|       2 |          2 |     28    |
|       3 |          3 |     22    |
|       4 |          4 |     4     |

deliveries
| id      | kg         | classid   |
|       1 |      120   |     1     |
|       2 |      80    |     1     |
|       3 |      20    |     1     |
|       4 |      200   |     2     |
|       5 |      150   |     3     |
|       6 |      14    |     2     |
school
| id