高手看看这个用SQL怎样实现？(100分)

入库表 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;出库表 &nbsp; <br>入库日期 &nbsp; &nbsp;商品 &nbsp;数量 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 出库日期 &nbsp; &nbsp;商品 &nbsp;数量<br>2008.12.12 &nbsp;牙膏 &nbsp; 1 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 2008.12.15 &nbsp;牙刷 &nbsp;1<br>2008.12.12 &nbsp;牙刷 &nbsp; 1 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 2008.12.15 &nbsp;牙膏 &nbsp;1<br>2008.12.13 &nbsp;牙刷 &nbsp; 1<br><br>怎样用SQL语句列出所有商品的库存数量？ 我用的是interbase数据库，下面的都不能用<br>不能from后接（select 等。。<br>select b.商品,sum(b.數量) as 庫存 from <br>(select 商品,數量 from 入庫表<br>union &nbsp;all <br>select 商品,數量*-1 as數量 from 出庫表) b group by 商品 &nbsp;<br>和<br>SELECT A.商品, A.总量-ISNULL(B.总量,0) AS 库存量 FROM<br>(SELECT 商品,SUM(数量) AS 总量 FROM 入库表 GROUP BY 商品) A <br>LEFT OUTER JOIN <br>SELECT (SELECT 商品,SUM(数量) AS 总量 FROM 出库表 GROUP BY 商品) B<br>ON A.商品=B.商品 &nbsp;<br><br>都用不了

select a.商品<br>sum(a.数量)-(select sum(b.数量) from 出 b where b.商品=a.商品)<br>from 入 a<br>group a.商品<br>当然都要加上日期

还是不正确，如果没有出过库的库存全成0了，怎样加个判断

select a.商品<br>sum(a.数量)-(select sum(b.数量) from 出 b where b.商品=a.商品)<br>from 入 a<br>group a.商品