161

我在弄清楚如何使用 LINQ to SQL 使用多个左外连接时遇到了一些麻烦。我了解如何使用一个左外连接。我正在使用 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
4

6 回答 6

249

这可能更干净（您不需要所有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

这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  
    }
于 2009-12-28T19:23:20.243 回答
49

无法访问 VisualStudio（我在我的 Mac 上），但使用来自http://bhaidar.net/cs/archive/2007/08/01/left-outer-join-in-linq-to的信息-sql.aspx看起来您可以执行以下操作：

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 }
于 2008-11-06T04:22:55.683 回答
22

我想出了如何使用 LINQ to SQL 在 VB.NET 中使用多个左外连接：

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
于 2008-11-06T15:01:05.347 回答
8

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
于 2012-09-17T01:28:22.380 回答
3

我认为您应该能够遵循本文中使用的方法。它看起来真的很难看，但我认为你可以做两次并得到你想要的结果。

我想知道这是否真的是你最好使用DataContext.ExecuteCommand(...)而不是转换为 linq 的情况。

于 2008-11-06T03:12:14.983 回答
0

我正在为我的应用程序使用这个 linq 查询。如果这符合您的要求，您可以参考这个。在这里，我加入了（左外连接）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()
于 2017-11-08T07:53:57.577 回答