sql server选择值取决于日期

I have 2 tables :

我有2张桌子:

1. documents

   (docId | date | value | currencyid | currencyRate | netvalue )
   1 | 2017/07/30 | 777 | EUR | 4.55 | 150.66
   2 | 2017/07/30 | 456 | EUR | 4.55 | 100.00
   3 | 2017/07/29 | 440 | RON | 1.00 | 440.00
   4 | 2017/07/28 | 999 | RON | 1.00 | 999.00
   (d