请各位帮忙优化一下这个查询语句
请各位帮忙优化一下这个代码，当BandingUnits为空时需要执行两遍查询，数据量多时会很慢，不知如何改进呀
select
isnull((Select
BandingUnits
From
dbo.MstProductUnits pu
Where
pu.ProductCode = fw.ProductCode
and pu.WarehouseCode = fw.WarehouseCode

),(Select
BandingUnits
From
dbo.MstProductUnits pu
Where
pu.ProductCode = fw.ProductCode
and pu.WarehouseCode = 0
)) as BandingUnits
from dbo.WrkForwarding as fw


[解决办法]
先分表，建立临时表，分块查找，范围缩小