两表分类汇总在汇总的问题

请教各位老师下面的代码是按物资编码汇总计算的，如果按两个字段汇总计算比如，物资编码、aa 又怎么写呢？谢谢

SQL = "SELECT fh.物资编码,fh.入库数量汇总-IIF(ISNULL(th.出库数量汇总),0,th.出库数量汇总) AS 库存 " _
        & "FROM (SELECT 物资编码, SUM(入库数量) AS 入库数量汇总 From 临时入库  GROUP BY 物资编码) AS fh LEFT JOIN " _
        & "(SELECT 物资编码, SUM(出库数量) AS 出库数量汇总 From 临时出库  GROUP BY 物资编码) AS th ON fh.物资编码 = th.物资编码"