求刷新进货单汇率的方法？

货币表: Curr (字段CurrNO, Adj_DD, ExchRate)
货币的汇率不定期做调整:
USD 2011-01-02 6.68
USD 2011-02-02 6.668
USD 2011-03-01 6.667

进货单表: Inventory(字段InvNo, Inv_DD, CurrNO, ExchRate 等等)

求一个SQL更新进货单中汇率, 条件: CurrNo 肯定要，取进货日期相近一个次调整汇率.

SQL code

update Curr set ExchRate=a.ExchRate
from Inventory a where Curr.CurrNo=a.CurrNo
and not exists
(select 1 from Inventory b where a.CurrNo=b.CurrNo and b.Inv_DD>a.Inv_DD)

SQL code

update Curr set ExchRate=a.ExchRate,Adj_DD=a.Inv_DD
from Inventory a where Curr.CurrNo=a.CurrNo
and not exists
(select 1 from Inventory b where a.CurrNo=b.CurrNo and b.Inv_DD>a.Inv_DD)