用脚或者用手

二者minus

select a.goodsid,(a.goodsnumber-b.goodsnumber)as remain
from a,b
where a.goodsid = b.goodsid

select 找什么
from 从哪找
where 条件

@落霞调皮小鬼

SELECT goodsid,sum(b)-sum(c) FROM(
SELECT a1.goodsid,SUM(a1.goodsnumber)b,0 AS c FROM A1
GROUP BY a1.goodsid
UNION
SELECT a2.goodsid,0 AS b,SUM(a2.goodsnumber)c FROM A2
GROUP BY a2.goodsid)

入库数据字段为正，出库为负 ，区分库存字段作为分组，最后sum 即可,举个例子：