LINQ |分组运算符|分组依据
原文:https://www . geesforgeks . org/linq-group-operator-group by/
在 LINQ，分组操作符选择包含公共属性的序列或集合的元素，并在一个组中为它们服务。或者换句话说，我们可以说分组运算符基于给定的键返回元素组。该组保存在一个特殊类型的集合中，该集合实现了 IGrouping 接口，其中 TKey 是创建组的密钥，TElement 是与分组密钥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，我们需要一个包含该序列中所有 Abc 的组。所以 Abc 是我们创建另一个包含 3 个 Abc 的组的关键。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:
- 基团
- 托勒密
分组运算符
GroupBy 运算符的工作类似于 SQL GroupBy 子句。它用于从给定的序列或集合中返回共享公共属性或键的元素组。每个组由 IGrouping 对象表示。
要点:
- 它支持 C# 和 VB.Net 语言中查询语法。如例 1 所示。
- 您也可以在 C# 查询中将 into 与 GroupBy 一起使用。into 关键字允许您继续查询，并可以执行更多的查询操作。或者换句话说，它是一个临时标识符，允许您执行额外的查询操作。
- 您也可以在 VB.Net 使用“分组”和“分组”。
- LINQ 查询以帮助 Select 或 Groupby 子句结束。
- 它还可以支持 C# 和 VB.Net 语言中的方法语法。如例 2 所示。
- 它同时出现在可查询类和可枚举类中。在这些类中，GroupBy 方法以八种不同的类型重载。
- 它通过使用延迟执行来实现。
- 键值可以是任何类型，如字符串、int、float、anonymous、bool 等。取决于用户的要求。
- 此运算符返回的组可以包含零个或多个与键值匹配的元素。
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<Employee> emp = new List<Employee>() {
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);
}
}
}
}
Output:
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
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<Employee> emp = new List<Employee>() {
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);
}
}
}
}
Output:
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
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