我试图得到两个表之间的差异总和，但我一直得到错误的结果

Table1                                           Table2      

| product  | quantity |                          | product  | quantity |
| -------- | -------- |                          | -------- | -------- |
| a        | 7        |                          | a        | 2        |
| b        | 8        |                          | b        | 4        |
| c        | 9        |                          | c        | 1        |
| c        | 7        |                          | c        | 3        |
| a        | 3        |                          | a        | 2        |
| b        | 4        |                          | b        | 3        |

我尝试了此查询，但得到了错误的值：

select table1.product, sum(table1.quantity) - sum(table2.quantity) as difference
from table1
  join table2 on table1.product = table2.product
group by table1.product,table2.product;

预期成果

表一
| 积|差|
| - ------|- ------|
| 项目a|六个|
| B|五个|
| （c）秘书长的报告|十二|