Linq to Entities Left Entities/ Subquery？

Question

我有一个查询(在linqpad中开发)：

DateTime currentDate = new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1);
DateTime previousMonth = currentDate.AddMonths(-1);
DateTime previousMonthForAveragePrices = currentDate.AddMonths(-2);


var help =  from cd in CostDrivers.OfType<Commodity>() 
                                  where cd.isActive == true && 
                                  cd.arePricesCalculatedAverage == false
                                  from cp in cd.CostDriverPrices where cp.priceDate
== currentDate 
                                  select new {cd.costDriverID, cd.costDriverName, cd.isUpdatedMonthly, cd.arePricesCalculatedAverage,
                                                cp.costDriverPriceID, priceDate = cp.priceDate,
                                                cp.price, previousPriceDate = from cpc in cd.CostDriverPrices where cpc.priceDate == previousMonth 
                                                select new {previousPrice = cpc.price, previousPriceDate = cpc.priceDate}};

help.Dump()；

我需要做的是返回所有costDrivers，而不管在给定日期(currentDate)是否存在价格记录。我应该指出的是，有人试图使用子查询来获取currentDate -1月份的另一个价格记录。我已尝试|| null等，不能继续。这是linq to entities。查询本身是有效的。它将只返回有价格的结果。谢谢!

谢谢。

Answer 1

这应该会对Left Outer Join in Linq-To-Entities有所帮助

此外，如果您愿意，我想您还可以向EF提交实际的SQL

http://msdn.microsoft.com/en-us/library/bb896272.aspx