查询表达式语法示例：联接运算符 (LINQ to DataSet)

项目
08/22/2008

更新：November 2007

联接是面向相互之间具有可导航关系的数据源（如关系数据库表）的查询中的一项重要操作。联接两个数据源就是将一个数据源中的对象与另一个数据源中具有相同公共属性的对象相关联。有关更多信息，请参见标准查询运算符概述。

本主题中的示例演示如何使用 GroupJoin 和 Join 方法以便使用查询表达式语法来查询 DataSet。

这些示例中所使用的 FillDataSet 方法已在向数据集中加载数据中指定。

本主题中的示例使用 AdventureWorks 示例数据库中的 Contact、Address、Product、SalesOrderHeader 和 SalesOrderDetail 表。

本主题中的示例使用下面的 using/Imports 语句：

Option Explicit On

Imports System
Imports System.Linq
Imports System.Linq.Expressions
Imports System.Collections.Generic
Imports System.Data
Imports System.Data.SqlClient
Imports System.Data.Common
Imports System.Globalization

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Data.Common;
using System.Globalization;

有关更多信息，请参见如何：在 Visual Studio 中创建 LINQ to DataSet 项目。

GroupJoin

示例

此示例对 SalesOrderHeader 和 GroupJoin 表执行 SalesOrderDetail 以得出每个客户的订单数。组联接等效于左外部联接，它返回第一个（左侧）数据源的每个元素（即使其他数据源中没有关联元素）。

' Fill the DataSet.
Dim ds As New DataSet()
ds.Locale = CultureInfo.InvariantCulture
' See the FillDataSet method in the Loading Data Into a DataSet topic.
FillDataSet(ds)

Dim orders = ds.Tables("SalesOrderHeader").AsEnumerable()
Dim details = ds.Tables("SalesOrderDetail").AsEnumerable()

Dim query = _
        From order In orders _
        Group Join detail In details _
            On order.Field(Of Integer)("SalesOrderID") _
            Equals detail.Field(Of Integer)("SalesOrderID") Into ords = Group _
        Select New With _
            { _
                .CustomerID = order.Field(Of Integer)("SalesOrderID"), _
                .ords = ords.Count() _
            }

For Each order In query
    Console.WriteLine("CustomerID: {0}  Orders Count: {1}", _
        order.CustomerID, order.ords)
Next

// Fill the DataSet.
DataSet ds = new DataSet();
ds.Locale = CultureInfo.InvariantCulture;
FillDataSet(ds);

var orders = ds.Tables["SalesOrderHeader"].AsEnumerable();
var details = ds.Tables["SalesOrderDetail"].AsEnumerable();

var query =
    from order in orders
    join detail in details
    on order.Field<int>("SalesOrderID")
    equals detail.Field<int>("SalesOrderID") into ords
    select new
    {
        CustomerID =
            order.Field<int>("SalesOrderID"),
        ords = ords.Count()
    };

foreach (var order in query)
{
    Console.WriteLine("CustomerID: {0}  Orders Count: {1}",
        order.CustomerID,
        order.ords);
}

示例

此示例对 Contact 和 SalesOrderHeader 表执行 GroupJoin。组联接等效于左外部联接，它返回第一个（左侧）数据源的每个元素（即使其他数据源中没有关联元素）。

' Fill the DataSet.
Dim ds As New DataSet()
ds.Locale = CultureInfo.InvariantCulture
' See the FillDataSet method in the Loading Data Into a DataSet topic.
FillDataSet(ds)

Dim contacts As DataTable = ds.Tables("Contact")
Dim orders As DataTable = ds.Tables("SalesOrderHeader")

Dim query = _
    From contact In contacts.AsEnumerable(), order In orders.AsEnumerable() _
    Where (contact.Field(Of Integer)("ContactID") = _
        order.Field(Of Integer)("ContactID")) _
    Select New With _
        { _
            .ContactID = contact.Field(Of Integer)("ContactID"), _
            .SalesOrderID = order.Field(Of Integer)("SalesOrderID"), _
            .FirstName = contact.Field(Of String)("FirstName"), _
            .Lastname = contact.Field(Of String)("Lastname"), _
            .TotalDue = order.Field(Of Decimal)("TotalDue") _
        }

For Each contact_order In query
    Console.Write("ContactID: " & contact_order.ContactID)
    Console.Write(" SalesOrderID: " & contact_order.SalesOrderID)
    Console.Write(" FirstName: " & contact_order.FirstName)
    Console.Write(" Lastname: " & contact_order.Lastname)
    Console.WriteLine(" TotalDue: " & contact_order.TotalDue)
Next

// Fill the DataSet.
DataSet ds = new DataSet();
ds.Locale = CultureInfo.InvariantCulture;
FillDataSet(ds);

DataTable contacts = ds.Tables["Contact"];
DataTable orders = ds.Tables["SalesOrderHeader"];

var query =
    from contact in contacts.AsEnumerable()
    join order in orders.AsEnumerable()
    on contact.Field<Int32>("ContactID") equals
    order.Field<Int32>("ContactID")
    select new
    {
        ContactID = contact.Field<Int32>("ContactID"),
        SalesOrderID = order.Field<Int32>("SalesOrderID"),
        FirstName = contact.Field<string>("FirstName"),
        Lastname = contact.Field<string>("Lastname"),
        TotalDue = order.Field<decimal>("TotalDue")
    };


foreach (var contact_order in query)
{
    Console.WriteLine("ContactID: {0} "
                    + "SalesOrderID: {1} "
                    + "FirstName: {2} "
                    + "Lastname: {3} "
                    + "TotalDue: {4}",
        contact_order.ContactID,
        contact_order.SalesOrderID,
        contact_order.FirstName,
        contact_order.Lastname,
        contact_order.TotalDue);
}

Join

示例

此示例对 SalesOrderHeader 和 SalesOrderDetail 表执行联接，以获取八月份的联机订单。

' Fill the DataSet.
Dim ds As New DataSet()
ds.Locale = CultureInfo.InvariantCulture
' See the FillDataSet method in the Loading Data Into a DataSet topic.
FillDataSet(ds)

Dim orders As DataTable = ds.Tables("SalesOrderHeader")
Dim details As DataTable = ds.Tables("SalesOrderDetail")


Dim query = _
    From order In orders.AsEnumerable() _
    Join detail In details.AsEnumerable() _
    On order.Field(Of Integer)("SalesOrderID") Equals _
            detail.Field(Of Integer)("SalesOrderID") _
    Where order.Field(Of Boolean)("OnlineOrderFlag") = True And _
            order.Field(Of DateTime)("OrderDate").Month = 8 _
    Select New With _
    { _
        .SalesOrderID = order.Field(Of Integer)("SalesOrderID"), _
        .SalesOrderDetailID = detail.Field(Of Integer)("SalesOrderDetailID"), _
        .OrderDate = order.Field(Of DateTime)("OrderDate"), _
        .ProductID = detail.Field(Of Integer)("ProductID") _
    }

For Each order In query
    Console.WriteLine(order.SalesOrderID & vbTab & _
        order.SalesOrderDetailID & vbTab & _
        order.OrderDate & vbTab & _
        order.ProductID)
Next

// Fill the DataSet.
DataSet ds = new DataSet();
ds.Locale = CultureInfo.InvariantCulture;
FillDataSet(ds);

DataTable orders = ds.Tables["SalesOrderHeader"];
DataTable details = ds.Tables["SalesOrderDetail"];

var query =
    from order in orders.AsEnumerable()
    join detail in details.AsEnumerable()
    on order.Field<int>("SalesOrderID") equals
        detail.Field<int>("SalesOrderID")
    where order.Field<bool>("OnlineOrderFlag") == true
    && order.Field<DateTime>("OrderDate").Month == 8
    select new
    {
        SalesOrderID =
            order.Field<int>("SalesOrderID"),
        SalesOrderDetailID =
            detail.Field<int>("SalesOrderDetailID"),
        OrderDate =
            order.Field<DateTime>("OrderDate"),
        ProductID =
            detail.Field<int>("ProductID")
    };


foreach (var order in query)
{
    Console.WriteLine("{0}\t{1}\t{2:d}\t{3}",
        order.SalesOrderID,
        order.SalesOrderDetailID,
        order.OrderDate,
        order.ProductID);
}