C#学习相关系列之Linq用法---group和join相关用法（三）

一、Group用法

         在C#的LINQ中，Grou将集合中的元素按照指定的键进行分组。Group方法返回一个IEnumerable>类型的集合，其中TKey表示分组的键类型，TElement表示集合中元素的类型。每个IGrouping对象表示一个分组，其中Key属性表示分组的键，而IEnumerable表示分组中的元素集合。

1、groupby的定义：

//Linq语句
var tt = from e in stuList group e by e.Math;
//lambda 表达式
var tt = stuList.GroupBy(p=>p.Math);

示例代码：

List stuList = new List()
{
    new Student_1(){ID=1,Name="John",Chinese=92,Math=88,English=92},
    new Student_1(){ID=2,Name="Mary",Chinese=87,Math=94,English=82},
    new Student_1(){ID=3,Name="KangKang",Chinese=89,Math=91,English=96},
    new Student_1(){ID=4,Name="Maria",Chinese=88,Math=65,English=94},
    new Student_1(){ID=5,Name="Ben",Chinese=70,Math=91,English=82},
};
//var tt = from e in stuList group e by e.Math;//linq语句

var tt = stuList.GroupBy(p=>p.Math);//lambda表达式
foreach (var item in tt)
{
    Console.WriteLine(item.Key);
    foreach (var e in item)
   {
        Console.WriteLine("    "+e.Name);
   }
}

运行结果为：

2、group...into...用法可以生成一个temp组

var tt = from e in stuList
            group new { e.Math, e.ID, e.Name } by e.Math  into temp 
            select new { temp.Key, value1 = temp.Select(p => new { p.ID, p.Name }), value2 = temp.Select(p => p.Name) };
//  var tt = stuList.GroupBy(p => p.Math, e => new { e.Math, e.ID, e.Name }).Select(temp => new { temp.Key, value1 = temp.Select(p => new { p.ID, p.Name }), value2 = temp.Select(p => p.Name) });
foreach (var item in tt)
{
    Console.WriteLine(item.Key);
    foreach (var e in item.value1)
    {
        Console.WriteLine(e.Name);
    }
}

运行结果为：

二、Join用法

Linq中的join用于将两个集合中的元素进行匹配，返回一个新的集合。

1、内部链接 Join  （join… in… on …equals…）

定义：

//使用查询语句 Linq
var list = from T1 in dataSource1
                join T2 in dataSource2 on T1.ID equals T2.ID
                select new {Name = T1.Name,ID=T1.ID,NickName = T2.NickName};


//使用查询方法 lambda 表达式
var list = dataSource1.Join(dataSource2,T1 => T1.ID,T2 => T2.ID, (T1,T2) => new {
        Name = T1.Name,
        ID=T1.ID,
        NickName = T2.NickName
});

代码示例：

List students = new List()
{
    new Student() { Id = 1, Name = "Tom" },
    new Student() { Id = 2, Name = "Jerry" },
    new Student() { Id = 3, Name = "Mike" }
};

List courses = new List()
{
    new Course() { Id = 1, Name = "Math", StudentId = 1 },
    new Course() { Id = 2, Name = "English", StudentId = 1 },
    new Course() { Id = 3, Name = "Math", StudentId = 2 },
    new Course() { Id = 4, Name = "Science", StudentId = 3 }
};

// 使用join将学生和课程进行匹配
var result = from s in students
                join c in courses on s.Id equals c.StudentId
                select new { StudentName = s.Name, CourseName = c.Name };
// 输出结果
foreach (var item in result)
{
    Console.WriteLine("{0}选了{1}课程", item.StudentName, item.CourseName);
}

运行结果为：

2、分组链接Groupjoin （join…in…on…equals…into…）

定义:

//lambda 表达式
var result = students.GroupJoin(courses, p => p.Id,s => s.StudentId,(p, e) => new { StudentName = p.Name, CourseName =e.Select(m=>m.Name)});

//linq 语句
var result = from left in students join right in courses on left.Id equals right.StudentId into temp select new { StudentName = left.Name, CourseName = temp.Select(m => m.Name) };
// temp 其实为course的分组

代码示例：

// 创建学生和课程集合
List students = new List()
{
    new Student() { Id = 1, Name = "Tom" },
    new Student() { Id = 2, Name = "Jerry" },
    new Student() { Id = 3, Name = "Mike" }
};

List courses = new List()
{
    new Course() { Id = 1, Name = "Math", StudentId = 1 },
    new Course() { Id = 2, Name = "English", StudentId = 1 },
    new Course() { Id = 3, Name = "Math", StudentId = 2 },
    new Course() { Id = 4, Name = "Science", StudentId = 3 }
};
// var result = students.GroupJoin(courses, p => p.Id,s => s.StudentId,(p, e) => new { StudentName = p.Name, CourseName =e.Select(m=>m.Name)});
var result = from left in students join right in courses on left.Id equals right.StudentId into temp select new { StudentName = left.Name, CourseName = temp.Select(m => m.Name) };
// temp 其实为course的分组
// 使用join将学生和课程进行匹配
//var result = from s in students
//                join c in courses on s.Id equals c.StudentId
//                select new { StudentName = s.Name, CourseName = c.Name };
// 输出结果
foreach (var item in result)
{
    Console.WriteLine("{0}选了课程", item.StudentName);
        foreach (var n in item.CourseName)
        {
            Console.WriteLine("课程为：{0}",n);
        }
}

运行结果为：

通过对比可以发现，join和groupjion的区别在于，join相当于左右一对一关系，例如：

左1=右1，左1=右2....这样一一列举，但groupjoin不一样，他是以左一为关键字，将左一的值生成一个list，例如 左1={右1，右2}...，在实际应用中根据需求选用不同的用法。