有两张表Table1，Table2。table1数据如下：
ID product amount
----------------------------------
1 A 10
2 A 12
3 B 5
4 B 11
5 A 3
table2数据如下：
ID product amount
----------------------------------
1 A 5
2 A 10
3 B 5
我想求出table1中A、B的总数，再减去table2中A、B的总数，然后生成新表
ID product amount
----------------------------------
1 A 10
2 A 11
这个SQL怎么写？
------解决方案--------------------
select a.product,(a.amount -b.amount ) from
(select product,sum(amount) amount from table1 group by product)a
(select product,sum(amount) amount from table2 group by product)b
where a.product=b.product