如何在实体框架查询中使用左连接？ [复制]答案

【问题标题】：how to use left join in entity framework query? [duplicate]如何在实体框架查询中使用左连接？ [复制]
【发布时间】：2014-03-11 23:42:55
【问题描述】：

我正在使用实体框架 4.5。我必须将 SQL 查询转换为实体查询：

SELECT Customer.CustCode, Invoice.InvoiceId, Invoice.BatchNumber, Invoice.InvoiceDate, Invoice.AdjustFlag, Invoice.InvoiceAmount,
       Invoice.InvoiceNote, Invoice.AmountPaid, Customer.BillingContact, Customer.BillingCompany, Customer.BillingStreet1,
       Customer.BillingStreet2, Customer.BillingCity, Customer.BillingState, Customer.BillingZip, [Order].PickupDate, [Order].OrderNumber,
       [Order].OrderTotal, [Order].ProNumber, [Order].PickupCompany, [Order].PickupCity, [Order].PickUpState, [Order].Dcompany,
       [Order].Dcity, [Order].Dstate, CONVERT(varchar(5), DeliverInTime, 114) AS DelInTime, [Order].PiecesWeight1, [Order].BaseRATE,
       [Order].POD, [Order].Requester, [Order].Po1, [Order].Po2, AccessorialCharge.Description,
       OrderDriverExtraCharge.AccessorialChargeDesc, OrderDriverExtraCharge.AccessorialChargeAmount, [Order].NormalDiscount,
       - 1 * [Order].DISCAmount AS DISCAmount
FROM (((Invoice INNER JOIN
       [Order] ON Invoice.InvoiceId = [Order].InvoiceId) INNER JOIN
       Customer ON Invoice.CustID = Customer.CustID) LEFT JOIN
       OrderDriverExtraCharge ON [Order].OrderNumberId = OrderDriverExtraCharge.OrderNumberId) LEFT JOIN
       AccessorialCharge ON OrderDriverExtraCharge.AccessorialChargeId = AccessorialCharge.AccessorialChargeId
where Invoice.InvoiceId = '1117782'

如果我将 OrderDriverExtraCharge.OrderNumberId) LEFT JOIN 更改为 OrderDriverExtraCharge.OrderNumberId) JOIN（简单连接）或内部连接，它不会显示正确的结果。

我试过这个：

from I in db.Invoices
join O in db.Orders on I.InvoiceId equals O.InvoiceId
join C in db.Customers on I.CustId equals C.CustId
join OD in db.OrderDriverExtraCharges on O.OrderNumberId equals OD.OrderNumberId
join AC in db.AccessorialCharges on OD.AccessorialChargeId equals AC.AccessorialChargeId
where I.InvoiceId == invoice.InvoiceId
select new PrintInvoiceViewModel()

但它没有显示所需的结果。请帮助我，如果它对我有用，我会标记你的答案。谢谢。

【问题讨论】：

查看此链接。它包含一个左外连接的例子。 itworksonmymachine.wordpress.com/2009/09/27/…

标签： c# asp.net-mvc linq entity-framework

【解决方案1】：

你可以这样做：

from I in db.Invoices
      join O in db.Orders on I.InvoiceId equals O.InvoiceId
      join C in db.Customers on I.CustId equals C.CustId
      from OD in db.OrderDriverExtraCharges
        .Where(w=>w.OrderNumberId==O.OrderNumberId).DefaultIfEmpty()
      from AC in db.AccessorialCharges 
        .Where(w=>w.AccessorialChargeId==OD.AccessorialChargeId).DefaultIfEmpty()
      where I.InvoiceId == invoice.InvoiceId
      select new PrintInvoiceViewModel()

【讨论】：

【解决方案2】：

您应该使用DefaultIfEmpty 方法，该方法返回IEnumerable<T> 的元素，如果序列为空，则返回默认值的单例集合：

from I in db.Invoices
join O in db.Orders on I.InvoiceId equals O.InvoiceId
join C in db.Customers on I.CustId equals C.CustId
join OD in db.OrderDriverExtraCharges on O.OrderNumberId equals OD.OrderNumberId into ODs 
from OD in ODs.DefaultIfEmpty()
join AC in db.AccessorialCharges on OD.AccessorialChargeId equals AC.AccessorialChargeId into ACs 
from AS in ACs.DefaultIfEmpty()
where I.InvoiceId == invoice.InvoiceId
select new PrintInvoiceViewModel()

【讨论】：