select 产品名称代码,数量-isnull((select sum(数量) from tb2 where 产品名称代码=a.产品名称代码),0)
-isnull((select sum(数量) from tb3 where 产品名称代码=a.产品名称代码),0)
from tb1 a

------解决方案--------------------
SQL code
select tb1.产品名称代码,sum(isnull(tb1.数量,0)) -sum(isnull(tb2.数量,0))-sum(isnull(tb3.数量,0)) -sum(isnull(tb4.数量,0)) from  tb1 
left join tb2 on tb1.产品名称代码 =tb2.产品名称代码 
left join tb3 on tb1.产品名称代码 =tb3.产品名称代码 
left join tb4 on tb1.产品名称代码 =tb4.产品名称代码

------解决方案--------------------
SQL code
select
  a.产品名称代码,
  a.数量-isnull(sum(b.数量),0)
from
   tb1 a,
   (select * from tb2 union  select * from tb3)b
where
   a.产品名称代码=b.产品名称代码
group by
   a.产品名称代码,a.数量

------解决方案--------------------
SQL code

--上边全写成tb1了
select 产品名称代码,数量=sum(数量*mark)
from
(select *,1 as mark from tb1
union all
select *,-1 as mark from tb2
union all
select *,-1 as mark from tb3) t group by 产品名称代码

------解决方案--------------------
SQL code
select a.产品名称代码,a.数量-isnull(sum(b.数量),0)
from tb1 a,(select * from tb2 union  select * from tb3)b
where  a.产品名称代码=b.产品名称代码
group by a.产品名称代码,a.数量