select Product, sum((case when BS='B' then 1 else -1 end) *Quantity)
from tb
group by Product

------解决方案--------------------

不好意思，刚才的反了,参考以下

SQL code
select Product, sum((case when BS='B' then -1 else 1 end) *Quantity)
from tb
group by Product

------解决方案--------------------
/*先分组*/
SELECT Product
WHERE Product=ProductRoot.Product) AS MyT FROM [tb] AS ProductRoot  
GROUP BY Product


/*设A 为一个产品的进入货数总数
 设B 为一个产品的进入货数总数
 设C  C=B-A
*/

/*A*/
SELECT SUM(Quantity) FROM [tb] WHERE Product=ProductRoot.Product AND BS='B'

/*B*/
SELECT SUM(Quantity) FROM [tb] WHERE Product=ProductRoot.Product AND BS='S'



结果:

/*代入A到分组中*/
SELECT Product, (SELECT SUM(Quantity) FROM [tb] WHERE Product=ProductRoot.Product AND BS='B')  
FROM [tb] AS ProductRoot  
GROUP BY Product

最终SQL语句

/*代入B到分组中结合A*/
SELECT Product, (SELECT SUM(Quantity) FROM [tb] WHERE Product=ProductRoot.Product AND BS='S')-(SELECT SUM(Quantity) FROM [tb] WHERE Product=ProductRoot.Product AND BS='B')  
FROM [tb] AS ProductRoot  
GROUP BY Product