Linq 到 Sql: 多个左外部连接

我遇到了一些麻烦，不知道如何使用 LINQtoSQL 来使用多个左外部连接。我知道如何使用一个左外连接。我用的是 VB.NET。下面是我的 SQL 语法。

T-SQL

SELECT
o.OrderNumber,
v.VendorName,
s.StatusName
FROM
Orders o
LEFT OUTER JOIN Vendors v ON
v.Id = o.VendorId
LEFT OUTER JOIN Status s ON
s.Id = o.StatusId
WHERE
o.OrderNumber >= 100000 AND
o.OrderNumber <= 200000
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小开

我认为你应该能够遵循的方法在 这个后使用。它看起来真的很丑，但我认为你可以做两次，并得到你想要的结果。

我想知道在这种情况下，您是否最好使用 DataContext.ExecuteCommand(...)而不是转换为 linq。

小开

不能访问 VisualStudio (我在我的 Mac 上) ，但是使用来自 http://bhaidar.net/cs/archive/2007/08/01/left-outer-join-in-linq-to-sql.aspx的信息，看起来你似乎可以做这样的事情:

var query = from o in dc.Orders
join v in dc.Vendors on o.VendorId equals v.Id into ov
from x in ov.DefaultIfEmpty()
join s in dc.Status on o.StatusId equals s.Id into os
from y in os.DefaultIfEmpty()
select new { o.OrderNumber, x.VendorName, y.StatusName }
小开

我弄明白了如何在 VB.NET 中使用 LINQtoSQL 来使用多个左外部连接:

Dim db As New ContractDataContext()


Dim query = From o In db.Orders _
Group Join v In db.Vendors _
On v.VendorNumber Equals o.VendorNumber _
Into ov = Group _
From x In ov.DefaultIfEmpty() _
Group Join s In db.Status _
On s.Id Equals o.StatusId Into os = Group _
From y In os.DefaultIfEmpty() _
Where o.OrderNumber >= 100000 And o.OrderNumber <= 200000 _
Select Vendor_Name = x.Name, _
Order_Number = o.OrderNumber, _
Status_Name = y.StatusName
小开
最佳答案

这可能更清洁(你不需要所有的 into语句) :

var query =
from order in dc.Orders
from vendor
in dc.Vendors
.Where(v => v.Id == order.VendorId)
.DefaultIfEmpty()
from status
in dc.Status
.Where(s => s.Id == order.StatusId)
.DefaultIfEmpty()
select new { Order = order, Vendor = vendor, Status = status }
//Vendor and Status properties will be null if the left join is null

下面是另一个左连接示例

var results =
from expense in expenseDataContext.ExpenseDtos
where expense.Id == expenseId //some expense id that was passed in
from category
// left join on categories table if exists
in expenseDataContext.CategoryDtos
.Where(c => c.Id == expense.CategoryId)
.DefaultIfEmpty()
// left join on expense type table if exists
from expenseType
in expenseDataContext.ExpenseTypeDtos
.Where(e => e.Id == expense.ExpenseTypeId)
.DefaultIfEmpty()
// left join on currency table if exists
from currency
in expenseDataContext.CurrencyDtos
.Where(c => c.CurrencyID == expense.FKCurrencyID)
.DefaultIfEmpty()
select new
{
Expense = expense,
// category will be null if join doesn't exist
Category = category,
// expensetype will be null if join doesn't exist
ExpenseType = expenseType,
// currency will be null if join doesn't exist
Currency = currency
}
小开

在 VB.NET 中使用函数,

Dim query = From order In dc.Orders
From vendor In
dc.Vendors.Where(Function(v) v.Id = order.VendorId).DefaultIfEmpty()
From status In
dc.Status.Where(Function(s) s.Id = order.StatusId).DefaultIfEmpty()
Select Order = order, Vendor = vendor, Status = status
小开

我正在为我的应用程序使用这个 linq 查询。如果这个符合你的要求，你可以参考这个。在这里，我已经联接(左外联接)与3个表。

 Dim result = (From csL In contractEntity.CSLogin.Where(Function(cs) cs.Login = login AndAlso cs.Password = password).DefaultIfEmpty
From usrT In contractEntity.UserType.Where(Function(uTyp) uTyp.UserTypeID = csL.UserTyp).DefaultIfEmpty ' <== makes join left join
From kunD In contractEntity.EmployeeMaster.Where(Function(kunDat) kunDat.CSLoginID = csL.CSLoginID).DefaultIfEmpty
Select New With {
.CSLoginID = csL.CSLoginID,
.UserType = csL.UserTyp}).ToList()

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