LINQ |分组运算符|通过…分组

在LINQ中，分组运算符选择包含公共属性的序列或集合的元素，然后将它们作为一个组来提供服务。换句话说，我们可以说分组运算符基于给定的键返回元素组。该组保存在一种特殊的集合中，该集合实现IGrouping 接口，其中TKey是创建该组的键，而TElement是与该组键值对等的元素的集合。例如，一个序列包含6个元素，即Abc，cvd，Abc，Abc，ert，Por，…。现在我们想要一个包含该序列中所有Abc的组。因此， Abc是创建包含3个Abc的另一个组的关键。如下图所示：

标准查询运算符包含2种不同类型的分组运算符：

1. 通过…分组
2. 去查查看

GroupBy运算符

GroupBy运算符的工作方式类似于SQL GroupBy子句。它用于返回共享给定序列或集合中的公共属性或键的元素组。每个组均由IGrouping 对象表示。

重要事项：

• 它支持C＃和VB.Net语言的查询语法。如示例1所示。
• 您也可以在C＃查询中与GroupBy一起使用。使用in关键字可以继续查询，并可以执行更多查询操作。换句话说，它是一个临时标识符，可让您执行其他查询操作。
• 您还可以在VB.Net中将Into Group与GroupBy一起使用。
• LINQ查询以帮助Select或Groupby子句结束。
• 它还可以支持C＃和VB.Net语言的方法语法。如示例2所示。
• 它同时存在于Queryable和Enumerable类中。在这些类中，GroupBy方法在8种不同类型中被重载。
• 它是通过使用延迟执行来实现的。
• 键值可以是字符串，整数，浮点数，匿名，布尔等任意类型，具体取决于用户的要求。
• 该运算符返回的组可以包含零个或多个与键值匹配的元素。

范例1：

// C# program to divide the employees
// in groups according to their salary
using System;
using System.Linq;
using System.Collections.Generic;
  
// Employee details
public class Employee {
  
    public int emp_id
    {
        get;
        set;
    }
  
    public string emp_name
    {
        get;
        set;
    }
  
    public string emp_gender
    {
        get;
        set;
    }
  
    public string emp_hire_date
    {
        get;
        set;
    }
  
    public int emp_salary
    {
        get;
        set;
    }
}
  
class GFG {
  
    // Main method
    static public void Main()
    {
        List emp = new List() {
  
            new Employee() {emp_id = 209, emp_name = "Anjita", emp_gender = "Female",
                                    emp_hire_date = "12/3/2017", emp_salary = 20000},
  
            new Employee() {emp_id = 210, emp_name = "Soniya", emp_gender = "Female",
                                    emp_hire_date = "22/4/2018", emp_salary = 30000},
  
            new Employee() {emp_id = 211, emp_name = "Rohit", emp_gender = "Male",
                                  emp_hire_date = "3/5/2016", emp_salary = 40000},
  
            new Employee() {emp_id = 212, emp_name = "Supriya", emp_gender = "Female",
                                      emp_hire_date = "4/8/2017", emp_salary = 40000},
  
            new Employee() {emp_id = 213, emp_name = "Anil", emp_gender = "Male",
                                emp_hire_date = "12/1/2016", emp_salary = 40000},
  
            new Employee() {emp_id = 214, emp_name = "Anju", emp_gender = "Female",
                                  emp_hire_date = "17/6/2015", emp_salary = 50000},
        };
  
        // Query to divide the employees
        // in the groups according to 
        // their salary using GroupBy 
        // operator in query syntax
        var res = from e in emp
                    group e by e.emp_salary;
  
        foreach(var val in res)
        {
              
            // Here salary is the key value
            Console.WriteLine("Group By Salary: {0}", val.Key);
  
            // Display name of the employees
            // Inner collection according to
            // the key value
            foreach(Employee e in val)
            {
                Console.WriteLine("Employee Name: {0}",
                                           e.emp_name);
            }
        }
    }
}

输出：

Group By Salary: 20000
Employee Name: Anjita
Group By Salary: 30000
Employee Name: Soniya
Group By Salary: 40000
Employee Name: Rohit
Employee Name: Supriya
Employee Name: Anil
Group By Salary: 50000
Employee Name: Anju

范例2：

// C# program to divide the employees
// in the groups according to their
// language
using System;
using System.Linq;
using System.Collections.Generic;
  
// Employee details
public class Employee {
  
    public int emp_id
    {
        get;
        set;
    }
  
    public string emp_name
    {
        get;
        set;
    }
  
    public string emp_gender
    {
        get;
        set;
    }
  
    public string emp_hire_date
    {
        get;
        set;
    }
  
    public int emp_salary
    {
        get;
        set;
    }
    public string emp_lang
    {
        get;
        set;
    }
}
  
class GFG {
  
    // Main method
    static public void Main()
    {
        List emp = new List() {
  
            new Employee() {emp_id = 209, emp_name = "Anjita", emp_gender = "Female",
                 emp_hire_date = "12/3/2017", emp_salary = 20000, emp_lang = "Ruby"},
  
            new Employee() {emp_id = 210, emp_name = "Soniya", emp_gender = "Female",
                 emp_hire_date = "22/4/2018", emp_salary = 30000, emp_lang = "Java"},
  
            new Employee() {emp_id = 211, emp_name = "Rohit", emp_gender = "Male",
               emp_hire_date = "3/5/2016", emp_salary = 40000, emp_lang = "Perl"},
  
            new Employee() {emp_id = 212, emp_name = "Supriya", emp_gender = "Female",
                     emp_hire_date = "4/8/2017", emp_salary = 40000, emp_lang = "Java"},
  
            new Employee() {emp_id = 213, emp_name = "Anil", emp_gender = "Male",
               emp_hire_date = "12/1/2016", emp_salary = 40000, emp_lang = "C#"},
  
            new Employee() {emp_id = 214, emp_name = "Anju", emp_gender = "Female",
                 emp_hire_date = "17/6/2015", emp_salary = 50000, emp_lang = "C#"},
        };
  
        // Query to divide the employees in 
        // the groups according to their 
        // language using GroupBy method in
        // the method syntax
        var res = emp.GroupBy(e => e.emp_lang);
  
        foreach(var val in res)
        {
              
            // Here language is the key value
            Console.WriteLine("Group By Language: {0}", val.Key);
  
            // Display name of the employees
            // Inner collection according to
            // the key value
            foreach(Employee e in val)
            {
                Console.WriteLine("Employee Name: {0}", e.emp_name);
            }
        }
    }
}

输出：

Group By Language: Ruby
Employee Name: Anjita
Group By Language: Java
Employee Name: Soniya
Employee Name: Supriya
Group By Language: Perl
Employee Name: Rohit
Group By Language: C#
Employee Name: Anil
Employee Name: Anju