使用 LINQ 连接多个数据源的 C# 程序

LINQ 被称为语言集成查询，它是在 .NET 3.5 中引入的。它为 .NET 语言提供了创建查询以从数据源检索数据的能力。在本文中，我们将讨论如何使用 LINQ 连接多个数据源。这里的数据源是指列表。所以我们使用列表集合创建三个包含学生详细信息的数据源，然后根据 id 连接数据，这在所有列表中都使用 join 关键字。

句法：

from iterator1 in data1

join iterator2 in data2

on iterator1.column_name equals iterator2.column_name

join iterator3 in data3

on iterator1.column_name equals iterator3.column_name

————————————————–

join iteratorn in datan

on iterator1.column_name equals iteratorn.column_name

编程需要懂一点英语

其中 data 是数据源列表，迭代器用于从特定数据源获取数据

返回：它将根据比较的 column_names 返回匹配的行。

例子：

Input:

Student
new Student{id = 7058, name = "sravan kumar", dept_id = 1, add_id = 21},
new Student{id = 7059, name = "jyothika", dept_id = 2, add_id = 22},
new Student{id = 7072, name = "harsha", dept_id = 1, add_id = 22},
new Student{id = 7076, name = "khyathi", dept_id = 4, add_id = 27},
   
Department
new Department{dept_id = 1, dept_name = "CSE"},
new Department{dept_id = 2, dept_name = "CSE"},
new Department{dept_id = 3, dept_name = "IT"},  
 
Address
new Address{add_id = 21, address_name = "hyd"},
new Address{add_id = 22, address_name = "railu-peta"},
new Address{add_id = 24, address_name = "chenchu-peta"},
             
Output:
ID: 7058--> Name: sravan kumar--> Department: CSE--> Address: hyd
ID: 7059--> Name: jyothika--> Department: CSE--> Address: railu-peta
ID: 7072--> Name: harsha--> Department: CSE--> Address: railu-peta

方法

1.通过声明变量，使用名为 Student、Department 和 Address 的列表创建三个数据源。

2.向这些列表添加值。

3.根据学生id、部门id、地址id进行join。

var result = (from stu in students

join dept in departments on stu.dept_id equals dept.dept_id

join add in addresses on stu.add_id equals add.add_id).ToList();

4.使用 select() 方法选择数据。

select new
{
    ID = stu.id,
    Name = stu.name,
    DeptName = dept.dept_name,
    address = add.address_name
}

5.显示每个循环的使用。

foreach (var e in result)
{
    Console.WriteLine("\tID: " + e.ID + "--> Name: " + 
                               e.Name + "--> Department: " + 
                           e.DeptName + "--> Address: " + e.address);
}

例子：

C#

// C# program to join multiple data sources
// Using LINQ
using System;
using System.Linq;
using System.Collections.Generic;
  
// Variables for Student list
public class Student
{
    public int id;
    public string name;
    public int dept_id;
    public int add_id;
}
  
// Variables for Department list
public class Department
{
    public int dept_id;
    public string dept_name;
}
  
// Variables for Address list
public class Address 
{
    public int add_id;
    public string address_name;
}
  
class GFG{
  
// Driver code   
static void Main(string[] args)
{
      
    // Enter data for Student list
    List students = new List()
    {
        new Student{ id = 7058, name = "sravan kumar",
                     dept_id = 1, add_id = 21 },
        new Student{ id = 7059, name = "jyothika",
                     dept_id = 2, add_id = 22 },
        new Student{ id = 7072, name = "harsha",
                     dept_id = 1, add_id = 22 },
        new Student{ id = 7076, name = "khyathi",
                     dept_id = 4, add_id = 27 },
    };
  
    List departments = new List()
    {
        new Department{ dept_id = 1, dept_name = "CSE" },
        new Department{ dept_id = 2, dept_name = "CSE" },
        new Department{ dept_id = 3, dept_name = "IT " },
    };
  
    List addresses = new List()
    {
        new Address{ add_id = 21, address_name = "hyd" },
        new Address{ add_id = 22, address_name = "railu-peta" },
        new Address{ add_id = 24, address_name = "chenchu-peta" },
    };
  
    // Join the students and other two tables
    var result = (from stu in students
  
                join dept in departments on stu
                .dept_id equals dept
                .dept_id
                  
                join add in addresses on stu
                .add_id equals add.add_id
                  
                select new 
                {
                    ID = stu.id, Name = stu.name,
                    DeptName = dept.dept_name,
                    address = add.address_name 
                      
                }).ToList();
                  
    // Display the result
    foreach(var e in result)
    {
        Console.WriteLine("\tID: " + e.ID + "--> Name: " + 
                                   e.Name + "--> Department: " + 
                               e.DeptName + "--> Address: " + e.address);
    }
}
}

输出：

ID: 7058--> Name: sravan kumar--> Department: CSE--> Address: hyd
ID: 7059--> Name: jyothika--> Department: CSE--> Address: railu-peta
ID: 7072--> Name: harsha--> Department: CSE--> Address: railu-peta