关于c＃：如何在实体框架查询中使用左连接？


how to use left join in entity framework query?

本问题已经有最佳答案，请猛点这里访问。

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：

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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改为OrderDriverExtraCharge.OrderNumberId) JOIN(简单连接)或内部连接，则不会显示正确的结果。

我尝试过：

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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。请帮帮我，如果你的回答对我有用的话，我会记下来的。谢谢您。

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您可以这样做：

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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()

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

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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()