LINQ Group Join

17 Mar 2025 | 阅读 2 分钟

在 LINQ 中，带有 'into' 表达式的 Join 子句称为 Group join。在 LINQ 中，Group join 生成对象元素的序列，该序列基于左右集合中的匹配元素。

如果从右侧集合中没有找到与左侧集合匹配的元素，则 join 子句将返回一个空数组。在 LINQ 中，Group join 等价于内部等值连接，只是元素的组织方式是分组的。

LINQ Group Join 的语法

使用 LINQ Group Join 根据我们的需求从给定集合中获取匹配元素的语法。

var result = from d in objDept
join e in objEmp on d.DepId equals e.DeptId into empDept
select new
{
    DepartmentName = d.DepName,
    Employees = from emp2 in empDept
    orderby emp2.Name
     select emp2
};

从上面的语法可以看出，我们使用带有 'into' 表达式的 Join 子句来获取集合中的匹配元素。

LINQ Group Join 的示例

这是一个 LINQ Group Join 的示例，用于根据所需的条件从集合中获取匹配的元素。

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

namespace ConsoleApp1
{
    class Programme2
    {
        static void Main(string[] args)
        {
`//Create an object 'objDept1' and add some values in the list of the 'Department'
            List<Department> objDept1 = new List<Department>
    (){
    //insert the data in the Department class
    new Department{DepId=1,DepName="Software"},
    new Department{DepId=2,DepName="Finance"},
    new Department{DepId=3,DepName="Health"}
    };
    //Create an object 'objEmp1' and add some values in the list of the 'Employee'
    List<Employee>objEmp1 = new List<Employee>()
            {
    //add the data in the Employee class
            new Employee { EmpId=1,Name = "Devansh", DeptId=1 },
            new Employee { EmpId=2,Name = "Shubham", DeptId=1 },
            new Employee { EmpId=3,Name = "Aman", DeptId=2 },
            new Employee { EmpId=4,Name = "Raman", DeptId =2},
            new Employee { EmpId=5,Name = "Madhav"}
            };
    //used the linq queries to group the elements of both the table by using the join clause based on the name of the department
            var result = from d in objDept1
            join e in objEmp1 on d.DepId equals e.DeptId into empDept
            select new
            {
            DepartmentName = d.DepName,
            Employees = from emp2 in empDept
            orderby emp2.Name
            select emp2
            };
            int totalItems = 0;
            foreach (var empGroup in result)
            {
            Console.WriteLine(empGroup.DepartmentName);
            foreach (var item in empGroup.Employees)
            {
            totalItems++;
            Console.WriteLine("    {0}", item.Name);
            }
            }
            Console.ReadLine();
            }
            }
    //create the class Department and Employee having the different information
            class Department
            {
            public int DepId { get; set; }
            public string DepName { get; set; }
            }
            class Employee
            {
            public int EmpId { get; set; }
            public string Name { get; set; }
            public int DeptId { get; set; }
            }
            }

在上面的示例中，我们使用带有 'into' 表达式的 Join 子句，并且我们根据部门名称对元素进行分组。

输出