--用一条SQL语句求出A,B的出库总量。
select mc,sum(s1) as total from tb_sell group by mc

--用一条SQL语句求出A,B各剩下多少，要求显示产品名称。

select pname,s1-total from tb_product,(select mc,sum(s1) as total from tb_sell group by mc)tmp
where tb_product.mc=tmp.mc

--查询出A、B的库存总量和出库总量、显示在一个表中，要求显示产品名称
select * from tb_product inner join (select mc,sum(s1) as total from tb_sell group by mc)tmp
on tb_product.mc=tmp.mc

---------------------------------------------
mc   total
---- -----------
A    333
B    1976

(2 行受影响)

pname                
-------------------- -----------
产品一                  664
产品二                  2567

(2 行受影响)

mc   pname                s1          mc   total
---- -------------------- ----------- ---- -----------
A    产品一                  997         A    333
B    产品二                  4543        B    1976

(2 行受影响)