在LINQ中，Join运算符用于将两个数据源集成到一个共享某些公共属性的数据源中。例如，在银行中，经理有两个列表，第一个列表包含个人详细信息，另一个列表包含房屋贷款详细信息。现在，经理想要创建一个列表，其中包含那些借了房屋贷款并属于同一地方的人的名字，因此他使用join子句来创建这种类型的列表。
join子句始终使用两个数据源，该数据源中存在的元素必须包含一些属性，以便它可以与其他数据源进行比较。 join子句的结果取决于所使用的join子句的类型。联接的最常见类型是：
- 内部联接
- 交叉连接
- 左外连接
- 团体加入
内部联接
在LINQ中，内部联接用于提供结果，该结果仅包含来自第一数据源的元素，这些元素仅在第二数据源中出现一次。并且，如果第一个数据源的元素没有匹配的元素，则它将不会出现在结果数据集中。这里的Join和Inner join相同。
_0.jpg)
重要事项：
- 所有通过join子句执行的联接都是等联接。
- 它支持C＃和VB.Net语言的查询语法。
句法：
join … in … on … equals … - 它支持C＃和VB.Net语言的方法语法。在这里，join方法以两种不同的方式被重载：
- 加入
，IEnumerable ，Func - Join
，IEnumerable ，Func )： 用于比较键。
- 加入
- Queryable和Enumerable类中都存在Join方法。
- Join的工作类似于SQL中的内部联接。
- join子句根据使用equals关键字指定的键比较数据源，此处==无效。
范例1：
// C# program to illustrate the concept
// of inner join in Query Syntax
using System;
using System.Linq;
using System.Collections.Generic;
// Employee details
public class Employee1 {
public int emp_id
{
get;
set;
}
public string emp_name
{
get;
set;
}
public string emp_lang
{
get;
set;
}
}
// Employee department details
public class Employee2 {
public int emp_id
{
get;
set;
}
public string emp_dept
{
get;
set;
}
public int emp_salary
{
get;
set;
}
}
class GFG {
// Main method
static public void Main()
{
List emp1 = new List() {
new Employee1() {emp_id = 300, emp_name = "Anu",
emp_lang = "C#"},
new Employee1() {emp_id = 301, emp_name = "Mohit",
emp_lang = "C"},
new Employee1() {emp_id = 302, emp_name = "Sona",
emp_lang = "Java"},
new Employee1() {emp_id = 303, emp_name = "Lana",
emp_lang = "Java"},
new Employee1() {emp_id = 304, emp_name = "Lion",
emp_lang = "C#"},
new Employee1() {emp_id = 305, emp_name = "Ramona",
emp_lang = "Java"},
};
List emp2 = new List() {
new Employee2() {emp_id = 300, emp_dept = "Designing",
emp_salary = 23000},
new Employee2() {emp_id = 301, emp_dept = "Developing",
emp_salary = 40000},
new Employee2() {emp_id = 302, emp_dept = "HR",
emp_salary = 50000},
new Employee2() {emp_id = 303, emp_dept = "Designing",
emp_salary = 60000},
};
// Query to find the name and
// the salary of the employees
// Using Inner Join
var res = from e1 in emp1
join e2 in emp2
on e1.emp_id equals e2.emp_id
select new
{
Emp_Name = e1.emp_name,
Emp_Salary = e2.emp_salary
};
// Display result
Console.WriteLine("Employee and their Salary: ");
foreach(var val in res)
{
Console.WriteLine("Employee Name: {0} Salary: {1}",
val.Emp_Name, val.Emp_Salary);
}
}
}
输出：
Employee and their Salary:
Employee Name: Anu Salary: 23000
Employee Name: Mohit Salary: 40000
Employee Name: Sona Salary: 50000
Employee Name: Lana Salary: 60000
范例2：
// C# program to illustrate the concept
// of inner join in Method Syntax
using System;
using System.Linq;
using System.Collections.Generic;
// Employee details
public class Employee1 {
public int emp_id
{
get;
set;
}
public string emp_name
{
get;
set;
}
public string emp_lang
{
get;
set;
}
}
// Employee department details
public class Employee2 {
public int emp_id
{
get;
set;
}
public string emp_dept
{
get;
set;
}
public int emp_salary
{
get;
set;
}
}
class GFG {
// Main method
static public void Main()
{
List emp1 = new List() {
new Employee1() {emp_id = 300, emp_name = "Anu",
emp_lang = "C#"},
new Employee1() {emp_id = 301, emp_name = "Mohit",
emp_lang = "C"},
new Employee1() {emp_id = 302, emp_name = "Sona",
emp_lang = "Java"},
new Employee1() {emp_id = 303, emp_name = "Lana",
emp_lang = "Java"},
new Employee1() {emp_id = 304, emp_name = "Lion",
emp_lang = "C#"},
new Employee1() {emp_id = 305, emp_name = "Ramona",
emp_lang = "Java"},
};
List emp2 = new List() {
new Employee2() {emp_id = 300, emp_dept = "Designing",
emp_salary = 23000},
new Employee2() {emp_id = 301, emp_dept = "Developing",
emp_salary = 40000},
new Employee2() {emp_id = 302, emp_dept = "HR",
emp_salary = 50000},
new Employee2() {emp_id = 303, emp_dept = "Designing",
emp_salary = 60000},
};
// Query to find the name and
// the department of the employees
// Using Join Method
var res = emp1.Join(emp2,
e1 => e1.emp_id,
e2 => e2.emp_id,
(e1, e2) => new {
EmployeeName = e1.emp_name,
EmployeeDepartment = e2.emp_dept });
// Display result
Console.WriteLine("Employee Name and their Department:");
foreach(var val in res)
{
Console.WriteLine("Employee Name: {0} Department: {1}",
val.EmployeeName, val.EmployeeDepartment);
}
}
}
输出：
Employee Name and their Department:
Employee Name: Anu Department: Designing
Employee Name: Mohit Department: Developing
Employee Name: Sona Department: HR
Employee Name: Lana Department: Designing