LINQ – 数据集

数据集在内存中提供了非常有用的数据表示，并用于各种基于数据的应用程序。LINQ to Dataset 作为 LINQ to ADO.NET 的一项技术，有助于以轻松的方式对 Dataset 的数据执行查询并提高生产力。

LINQ To Dataset 介绍

LINQ to Dataset 使开发人员的查询任务变得简单。他们不需要用特定的查询语言编写查询，而是可以用编程语言编写。LINQ to Dataset 还可用于查询从多个数据源合并数据的位置。这也不需要任何 LINQ 提供程序，就像用于访问内存集合中的数据的 LINQ to SQL 和 LINQ to XML 一样。

下面是一个简单的 LINQ to Dataset 查询示例，其中首先获取数据源，然后用两个数据表填充数据集。在两个表之间建立关系，并通过 join 子句针对两个表创建 LINQ 查询。最后，foreach 循环用于显示所需的结果。

C＃

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LINQtoDataset {
   class Program {
      static void Main(string[] args) {
      
         string connectString = System.Configuration.ConfigurationManager.ConnectionStrings["LinqToSQLDBConnectionString"].ToString();

         string sqlSelect = "SELECT * FROM Department;" + "SELECT * FROM Employee;";

         // Create the data adapter to retrieve data from the database
         SqlDataAdapter da = new SqlDataAdapter(sqlSelect, connectString);
        
         // Create table mappings
         da.TableMappings.Add("Table", "Department");
         da.TableMappings.Add("Table1", "Employee");

         // Create and fill the DataSet
         DataSet ds = new DataSet();
         da.Fill(ds);

         DataRelation dr = ds.Relations.Add("FK_Employee_Department",
                           ds.Tables["Department"].Columns["DepartmentId"],
                           ds.Tables["Employee"].Columns["DepartmentId"]);

         DataTable department = ds.Tables["Department"];
         DataTable employee = ds.Tables["Employee"];

         var query = from d in department.AsEnumerable()
                     join e in employee.AsEnumerable()
                     on d.Field<int>("DepartmentId") equals
                     e.Field<int>("DepartmentId")                        
                     select new {
                        EmployeeId = e.Field<int>("EmployeeId"),
                        Name = e.Field<string>("Name"),                            
                        DepartmentId = d.Field<int>("DepartmentId"),                            
                        DepartmentName = d.Field<string>("Name")
                     };

         foreach (var q in query) {
            Console.WriteLine("Employee Id = {0} , Name = {1} , Department Name = {2}",
               q.EmployeeId, q.Name, q.DepartmentName);
         }

         Console.WriteLine("\nPress any key to continue.");
         Console.ReadKey();
      }
   }
}

VB

Imports System.Data.SqlClient
Imports System.Linq

Module LinqToDataSet

   Sub Main()
   
      Dim connectString As String = System.Configuration.ConfigurationManager.ConnectionStrings("LinqToSQLDBConnectionString").ToString()

      Dim sqlSelect As String = "SELECT * FROM Department;" + "SELECT * FROM Employee;"
      Dim sqlCnn As SqlConnection = New SqlConnection(connectString)
      sqlCnn.Open()

      Dim da As New SqlDataAdapter
      da.SelectCommand = New SqlCommand(sqlSelect, sqlCnn)

      da.TableMappings.Add("Table", "Department")
      da.TableMappings.Add("Table1", "Employee")

      Dim ds As New DataSet()
      da.Fill(ds)

      Dim dr As DataRelation = ds.Relations.Add("FK_Employee_Department", ds.Tables("Department").Columns("DepartmentId"), ds.Tables("Employee").Columns("DepartmentId"))

      Dim department As DataTable = ds.Tables("Department")
      Dim employee As DataTable = ds.Tables("Employee")

      Dim query = From d In department.AsEnumerable()
                  Join e In employee.AsEnumerable() On d.Field(Of Integer)("DepartmentId") Equals
                  e.Field(Of Integer)("DepartmentId")
                  Select New Person With { _
                        .EmployeeId = e.Field(Of Integer)("EmployeeId"),
                        .EmployeeName = e.Field(Of String)("Name"),
                        .DepartmentId = d.Field(Of Integer)("DepartmentId"),
                        .DepartmentName = d.Field(Of String)("Name")
                  }

      For Each e In query
         Console.WriteLine("Employee Id = {0} , Name = {1} , Department Name = {2}", e.EmployeeId, e.EmployeeName, e.DepartmentName)
      Next

      Console.WriteLine(vbLf & "Press any key to continue.")
      Console.ReadKey()
	  
   End Sub
  
   Class Person
      Public Property EmployeeId As Integer
      Public Property EmployeeName As String
      Public Property DepartmentId As Integer
      Public Property DepartmentName As String
   End Class
   
End Module

编译并执行上述 C# 或 VB 代码时，会产生以下结果 –

Employee Id = 1, Name = William, Department Name = Account
Employee Id = 2, Name = Benjamin, Department Name = Account
Employee Id = 3, Name = Miley, Department Name = Sales

Press any key to continue.

使用 LinQ to Dataset 查询数据集

在开始使用 LINQ to Dataset 查询数据集之前，将数据加载到数据集至关重要，这是通过使用 DataAdapter 类或通过 LINQ to SQL 来完成的。使用 LINQ to Dataset 制定查询与使用 LINQ 和其他启用 LINQ 的数据源制定查询非常相似。

单表查询

在以下单表查询中，从 SalesOrderHeaderTtable 中收集所有在线订单，然后将订单 ID、订单日期以及订单编号显示为输出。

C＃

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LinqToDataset {
   class SingleTable {
      static void Main(string[] args) {
      
         string connectString = System.Configuration.ConfigurationManager.ConnectionStrings["LinqToSQLDBConnectionString"].ToString();

         string sqlSelect = "SELECT * FROM Department;";

         // Create the data adapter to retrieve data from the database
         SqlDataAdapter da = new SqlDataAdapter(sqlSelect, connectString);

         // Create table mappings
         da.TableMappings.Add("Table", "Department");           

         // Create and fill the DataSet
         DataSet ds = new DataSet();
         da.Fill(ds);

         DataTable department = ds.Tables["Department"];            

         var query = from d in department.AsEnumerable()                        
         select new {
            DepartmentId = d.Field<int>("DepartmentId"),
            DepartmentName = d.Field<string>("Name")
         };

         foreach (var q in query) {
            Console.WriteLine("Department Id = {0} , Name = {1}",
               q.DepartmentId, q.DepartmentName);
         }

         Console.WriteLine("\nPress any key to continue.");
         Console.ReadKey();
      }
   }
}

VB

Imports System.Data.SqlClient
Imports System.Linq

Module LinqToDataSet

   Sub Main()
   
      Dim connectString As String = System.Configuration.ConfigurationManager.ConnectionStrings("LinqToSQLDBConnectionString").ToString()

      Dim sqlSelect As String = "SELECT * FROM Department;"
      Dim sqlCnn As SqlConnection = New SqlConnection(connectString)
      sqlCnn.Open()

      Dim da As New SqlDataAdapter
      da.SelectCommand = New SqlCommand(sqlSelect, sqlCnn)

      da.TableMappings.Add("Table", "Department")
      Dim ds As New DataSet()
      da.Fill(ds)

      Dim department As DataTable = ds.Tables("Department")

      Dim query = From d In department.AsEnumerable()
      Select New DepartmentDetail With {
         .DepartmentId = d.Field(Of Integer)("DepartmentId"),
            .DepartmentName = d.Field(Of String)("Name")
      }

      For Each e In query
         Console.WriteLine("Department Id = {0} , Name = {1}", e.DepartmentId, e.DepartmentName)
      Next

      Console.WriteLine(vbLf & "Press any key to continue.")
      Console.ReadKey()
   End Sub

   Public Class DepartmentDetail
      Public Property DepartmentId As Integer
      Public Property DepartmentName As String
   End Class
   
End Module

编译并执行上述 C# 或 VB 代码时，会产生以下结果 –

Department Id = 1, Name = Account
Department Id = 2, Name = Sales
Department Id = 3, Name = Pre-Sales
Department Id = 4, Name = Marketing

Press any key to continue.