一、Group用法

?????????在C#的LINQ中，Grou將集合中的元素按照指定的鍵進行分組。Group方法返回一個IEnumerable<IGrouping<TKey, TElement>>類型的集合，其中TKey表示分組的鍵類型，TElement表示集合中元素的類型。每個IGrouping<TKey, TElement>對象表示一個分組，其中Key屬性表示分組的鍵，而IEnumerable<TElement>表示分組中的元素集合。

1、groupby的定義：

//Linq語句
var tt = from e in stuList group e by e.Math;
//lambda 表達式
var tt = stuList.GroupBy(p=>p.Math);

示例代碼：

List<Student_1> stuList = new List<Student_1>()
{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語句var tt = stuList.GroupBy(p=>p.Math);//lambda表達式
foreach (var item in tt)
{Console.WriteLine(item.Key);foreach (var e in item){Console.WriteLine("    "+e.Name);}
}

運行結果為：

2、group...into...用法可以生成一個temp組

var tt = from e in stuListgroup 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);}
}

運行結果為：

二、Join用法

Linq中的join用于將兩個集合中的元素進行匹配，返回一個新的集合。

1、內部鏈接 Join? （join… in… on …equals…）

定義：

//使用查詢語句 Linq
var list = from T1 in dataSource1join T2 in dataSource2 on T1.ID equals T2.IDselect new {Name = T1.Name,ID=T1.ID,NickName = T2.NickName};//使用查詢方法 lambda 表達式
var list = dataSource1.Join(dataSource2,T1 => T1.ID,T2 => T2.ID, (T1,T2) => new {Name = T1.Name,ID=T1.ID,NickName = T2.NickName
});

代碼示例：

List<Student> students = new List<Student>()
{new Student() { Id = 1, Name = "Tom" },new Student() { Id = 2, Name = "Jerry" },new Student() { Id = 3, Name = "Mike" }
};List<Course> courses = new List<Course>()
{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將學生和課程進行匹配
var result = from s in studentsjoin c in courses on s.Id equals c.StudentIdselect new { StudentName = s.Name, CourseName = c.Name };
// 輸出結果
foreach (var item in result)
{Console.WriteLine("{0}選了{1}課程", item.StudentName, item.CourseName);
}

運行結果為：

2、分組鏈接Groupjoin （join…in…on…equals…into…）

定義:

//lambda 表達式
var result = students.GroupJoin(courses, p => p.Id,s => s.StudentId,(p, e) => new { StudentName = p.Name, CourseName =e.Select(m=>m.Name)});//linq 語句
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 其實為course的分組

代碼示例：

// 創建學生和課程集合
List<Student> students = new List<Student>()
{new Student() { Id = 1, Name = "Tom" },new Student() { Id = 2, Name = "Jerry" },new Student() { Id = 3, Name = "Mike" }
};List<Course> courses = new List<Course>()
{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 其實為course的分組
// 使用join將學生和課程進行匹配
//var result = from s in students
//                join c in courses on s.Id equals c.StudentId
//                select new { StudentName = s.Name, CourseName = c.Name };
// 輸出結果
foreach (var item in result)
{Console.WriteLine("{0}選了課程", item.StudentName);foreach (var n in item.CourseName){Console.WriteLine("課程為：{0}",n);}
}

運行結果為：

通過對比可以發現，join和groupjion的區別在于，join相當于左右一對一關系，例如：

左1=右1，左1=右2....這樣一一列舉，但groupjoin不一樣，他是以左一為關鍵字，將左一的值生成一個list，例如 左1={右1，右2}...，在實際應用中根據需求選用不同的用法。