灵巧；如何在多个条件下执行左外连接？答案

【问题标题】：LINQ; How to perform left outer join with multiple conditions?灵巧；如何在多个条件下执行左外连接？
【发布时间】：2013-04-19 12:53:44
【问题描述】：

我在模仿：

DB1 AS A LEFT OUTER JOIN
DB2 AS B 
ON A.[Currency Code] = B.[Currency Code] 
AND A.[Document Date] >= B.[Starting Date] 
AND A.[Document Date] <= B.[Ending Date]

这就是我现在拥有的：

from ledgers in ledgerEntries
join currency in currencyExchange
    on ledgers.CurrencyCode equals currency.CurrencyCode
        into c
from currencies in c.DefaultIfEmpty()
where
    ledgers.DocumentDate >= currencies.StartingDate
    && ledgers.DocumentDate <= currencies.EndingDate

我读过有关创建匿名类型并将它们设置为彼此相等的信息，但是当使用小于和大于比较日期时，这不起作用。

【问题讨论】：

标签： sql linq left-join

【解决方案1】：

看来this 的答案正是我想要的。

from ledgers in ledgerEntries
join currency in currencyExchange
    on ledgers.CurrencyCode equals currency.CurrencyCode
        into c
from currencies in c.Where(currency => currency.StartingDate <= ledgers.DocumentDate 
&& currency.EndingDate <= ledgers.DocumentDate).DefaultIfEmpty()

结合this的回答，我可以简化为：

from ledgers in ledgerEntries
from currencies in currencyExchange.Where(
    currency => currency.CurrencyCode == ledgers.CurrencyCode
    && currency.StartingDate <= ledgers.DocumentDate 
    && currency.EndingDate <= ledgers.DocumentDate).DefaultIfEmpty()

【讨论】：