LINQ - 数据集
数据集提供了一种非常有用的内存数据表示形式，并用于各种基于数据应用程序。LINQ to Dataset 作为 LINQ to ADO.NET 技术之一，方便以轻松的方式对数据集中的数据执行查询，并提高生产力。
LINQ To Dataset 简介
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 也可用于查询来自多个数据源的合并数据。这也不需要任何 LINQ 提供程序，就像 LINQ to SQL 和 LINQ to XML 用于访问内存集合中的数据一样。
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，并通过联接子句对这两个表创建 LINQ 查询。最后，使用 foreach 循环显示所需的结果。
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
当上述 C# 或 VB 代码编译并执行时，它会产生以下结果：
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，至关重要的是将数据加载到数据集，这可以通过使用 DataAdapter 类或 LINQ to SQL 来完成。使用 LINQ to Dataset 构建查询的方式与使用 LINQ 以及其他支持 LINQ 的数据源构建查询的方式非常相似。
单表查询
在下面的单表查询中，从 SalesOrderHeaderTtable 收集所有在线订单，然后显示订单 ID、订单日期以及订单号作为输出。
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
当上述 C# 或 VB 代码编译并执行时，它会产生以下结果：
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.
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