Linq order by、group by 和 order by 每组？

Question

我有一个看起来像这样的对象：

public class Student
{
    public string Name { get; set; } 
    public int Grade { get; set; }
}

我想创建以下查询：按学生姓名对成绩进行分组，按成绩对每个学生组进行排序，并按每组中的最高成绩对组进行排序。

所以结果应该是这样的：

John 100
John 80
Lucy 80
Lucy 50
Lucy 40
Eric 70
Eric 30

我创建了以下查询：

StudentsGrades.GroupBy(student => student.Name)
    .OrderBy(studentGradesGroup => studentGradesGroup.Max(student => student.Grade));

但这会返回

IEnumerable

IGrouping

，并且我无法对内部列表进行排序，除非我在另一个

foreach

查询中执行此操作并使用

AddRange

将结果添加到不同的列表。

有更漂亮的方法吗？

Answer 1

当然：

var query = grades.GroupBy(student => student.Name)
                  .Select(group => 
                        new { Name = group.Key,
                              Students = group.OrderByDescending(x => x.Grade) })
                  .OrderBy(group => group.Students.First().Grade);

请注意，订购后您可以只选择每组中的第一名，因为您已经知道第一个条目将获得最高分。

然后你可以用以下方式显示它们：

foreach (var group in query)
{
    Console.WriteLine("Group: {0}", group.Name);
    foreach (var student in group.Students)
    {
        Console.WriteLine("  {0}", student.Grade);
    }
}

Answer 2

无需投影的方法：

StudentsGrades.OrderBy(student => student.Name).
ThenBy(student => student.Grade);

Answer 3

我认为你想要一个额外的投影，将每个组映射到该组的排序版本：

.Select(group => group.OrderByDescending(student => student.Grade))

---

看起来您可能想要在此之后进行另一次展平操作，这将为您提供一系列学生而不是一系列组：

.SelectMany(group => group)

您始终可以将两者折叠成一个 单个

SelectMany

调用，一起进行投影和展平。

---

编辑： 正如 Jon Skeet 指出的那样，整体查询存在一定的低效率；对每个组进行排序所获得的信息不会用于对组本身进行排序。通过将每个组的排序移至组本身的排序之前，可以将 Max 查询避开为更简单的

First

查询。

Answer 4

public class Student 
    {
        public int Grade { get; set; }
        public string Name { get; set; }
        public override string ToString()
        {
            return string.Format("Name{0} : Grade{1}", Name, Grade);
        }
    }

class Program
{
    static void Main(string[] args)
    {

      List<Student> listStudents = new List<Student>();
      listStudents.Add(new Student() { Grade = 10, Name = "Pedro" });
      listStudents.Add(new Student() { Grade = 10, Name = "Luana" });
      listStudents.Add(new Student() { Grade = 10, Name = "Maria" });
      listStudents.Add(new Student() { Grade = 11, Name = "Mario" });
      listStudents.Add(new Student() { Grade = 15, Name = "Mario" });
      listStudents.Add(new Student() { Grade = 10, Name = "Bruno" });
      listStudents.Add(new Student() { Grade = 10, Name = "Luana" });
      listStudents.Add(new Student() { Grade = 11, Name = "Luana" });
      listStudents.Add(new Student() { Grade = 22, Name = "Maria" });
      listStudents.Add(new Student() { Grade = 55, Name = "Bruno" });
      listStudents.Add(new Student() { Grade = 77, Name = "Maria" });
      listStudents.Add(new Student() { Grade = 66, Name = "Maria" });
      listStudents.Add(new Student() { Grade = 88, Name = "Bruno" });
      listStudents.Add(new Student() { Grade = 42, Name = "Pedro" });
      listStudents.Add(new Student() { Grade = 33, Name = "Bruno" });
      listStudents.Add(new Student() { Grade = 33, Name = "Luciana" });
      listStudents.Add(new Student() { Grade = 17, Name = "Maria" });
      listStudents.Add(new Student() { Grade = 25, Name = "Luana" });
      listStudents.Add(new Student() { Grade = 25, Name = "Pedro" });

      listStudents.GroupBy(g => g.Name).OrderBy(g => g.Key).SelectMany(g => g.OrderByDescending(x => x.Grade)).ToList().ForEach(x => Console.WriteLine(x.ToString()));
    }
}

Answer 5

     var _items = from a in StudentsGrades
                  group a by a.Name;

     foreach (var _itemGroup in _items)
     {
        foreach (var _item in _itemGroup.OrderBy(a=>a.grade))
        {
           ------------------------
           --------------------------
        }
     }