select Insert_SingleNumber as 所属订单,Insert_GoodsName as 货物名称,Insert_GoodsModel as 规格型号,Insert_GoodsUnit as 单位,Insert_GoodsPrice as 单价,Insert_GoodsAmount as 数量 
from MIS_InsertInfo , MIS_Single
where InsertInfo.Insert_SingleNumber = MIS_Single.Single_Number and MIS_Single.Single_Number=321

------解决方案--------------------
貌似你后面那个表没什么用处,可直接更改为如下:
SQL code
select Insert_SingleNumber as 所属订单,Insert_GoodsName as 货物名称,Insert_GoodsModel as 规格型号,Insert_GoodsUnit as 单位,Insert_GoodsPrice as 单价,Insert_GoodsAmount as 数量 
from MIS_InsertInfo 
where Insert_SingleNumber = 321

------解决方案--------------------
in 的效率比较低，大数据量的话查询很慢
建议用exists

SQL code

select Insert_SingleNumber as 所属订单,Insert_GoodsName as 货物名称,Insert_GoodsModel as 规格型号,
Insert_GoodsUnit as 单位,Insert_GoodsPrice as 单价,Insert_GoodsAmount as 数量 
from MIS_InsertInfo 
where exists
(select Single_Number 
    from MIS_Single 
    where Single_Number=321
    and MIS_InsertInfo.Insert_SingleNumber = MIS_Single.Single_Number)