[转]Linq中GroupBy方法的使用总结

 Demo模型类：

public class StudentScore
{

    public int ID { set; get; }

    public string Name { set; get; }

    public string Course { set; get; }

    public int Score { set; get; }

    public string Term { set; get; }

}

Demo示例代码：

static void Main()
        {
            var lst = new List<StudentScore>
                          {
                              new StudentScore {ID = 1, Name = "张三", Term = "第一学期", Course = "Math", Score = 80},
                              new StudentScore {ID = 1, Name = "张三", Term = "第一学期", Course = "Chinese", Score = 90},
                              new StudentScore {ID = 1, Name = "张三", Term = "第一学期", Course = "English", Score = 70},
                              new StudentScore {ID = 2, Name = "李四", Term = "第一学期", Course = "Math", Score = 60},
                              new StudentScore {ID = 2, Name = "李四", Term = "第一学期", Course = "Chinese", Score = 70},
                              new StudentScore {ID = 2, Name = "李四", Term = "第一学期", Course = "English", Score = 30},
                              new StudentScore {ID = 3, Name = "王五", Term = "第一学期", Course = "Math", Score = 100},
                              new StudentScore {ID = 3, Name = "王五", Term = "第一学期", Course = "Chinese", Score = 80},
                              new StudentScore {ID = 3, Name = "王五", Term = "第一学期", Course = "English", Score = 80},
                              new StudentScore {ID = 4, Name = "赵六", Term = "第一学期", Course = "Math", Score = 90},
                              new StudentScore {ID = 4, Name = "赵六", Term = "第一学期", Course = "Chinese", Score = 80},
                              new StudentScore {ID = 4, Name = "赵六", Term = "第一学期", Course = "English", Score = 70},
                              new StudentScore {ID = 1, Name = "张三", Term = "第二学期", Course = "Math", Score = 100},
                              new StudentScore {ID = 1, Name = "张三", Term = "第二学期", Course = "Chinese", Score = 80},
                              new StudentScore {ID = 1, Name = "张三", Term = "第二学期", Course = "English", Score = 70},
                              new StudentScore {ID = 2, Name = "李四", Term = "第二学期", Course = "Math", Score = 90},
                              new StudentScore {ID = 2, Name = "李四", Term = "第二学期", Course = "Chinese", Score = 50},
                              new StudentScore {ID = 2, Name = "李四", Term = "第二学期", Course = "English", Score = 80},
                              new StudentScore {ID = 3, Name = "王五", Term = "第二学期", Course = "Math", Score = 90},
                              new StudentScore {ID = 3, Name = "王五", Term = "第二学期", Course = "Chinese", Score = 70},
                              new StudentScore {ID = 3, Name = "王五", Term = "第二学期", Course = "English", Score = 80},
                              new StudentScore {ID = 4, Name = "赵六", Term = "第二学期", Course = "Math", Score = 70},
                              new StudentScore {ID = 4, Name = "赵六", Term = "第二学期", Course = "Chinese", Score = 60},
                              new StudentScore {ID = 4, Name = "赵六", Term = "第二学期", Course = "English", Score = 70},
                          };

//分组，根据姓名，统计Sum的分数，统计结果放在匿名对象中。两种写法。 
            //第一种写法 
            Console.WriteLine("---------第一种写法");
            var studentSumScore_1 = (from l in lst
                                     group l by l.Name
                                     into grouped
                                     orderby grouped.Sum(m => m.Score)
                                     select new {Name = grouped.Key, Scores = grouped.Sum(m => m.Score)}).ToList();
            foreach (var l in studentSumScore_1)
            {
                Console.WriteLine("{0}:总分{1}", l.Name, l.Scores);
            }
  
            //第二种写法 
            Console.WriteLine("---------第二种写法");
            var studentSumScore_2 = lst.GroupBy(m => m.Name)
                .Select(k => new {Name = k.Key, Scores = k.Sum(l => l.Score)})
                .OrderBy(m => m.Scores).ToList();
            foreach (var l in studentSumScore_2)
            {
                Console.WriteLine("{0}:总分{1}", l.Name, l.Scores);
            }
  
            //分组，根据2个条件学期和课程，统计各科均分,统计结果放在匿名对象中。两种写法。 
            Console.WriteLine("---------第一种写法");
            var TermAvgScore_1 = (from l in lst
                                  group l by new {l.Term, l.Course}
                                  into grouped
                                  orderby grouped.Average(m => m.Score) ascending
                                  orderby grouped.Key.Term descending
                                  select
                                      new {grouped.Key.Term, grouped.Key.Course, Scores = grouped.Average(m => m.Score)})
                .ToList();
            foreach (var l in TermAvgScore_1)
            {
                Console.WriteLine("学期:{0},课程:{1},均分:{2}", l.Term, l.Course, l.Scores);
            }
            Console.WriteLine("---------第二种写法");
            var TermAvgScore_2 = lst.GroupBy(m => new {m.Term, m.Course})
                .Select(k => new {k.Key.Term, k.Key.Course, Scores = k.Average(m => m.Score)})
                .OrderBy(l => l.Scores).ThenByDescending(l => l.Term);
            foreach (var l in TermAvgScore_2)
            {
                Console.WriteLine("学期:{0},课程:{1},均分:{2}", l.Term, l.Course, l.Scores);
            }
  
            //分组，带有Having的查询，查询均分>80的学生 
            Console.WriteLine("---------第一种写法");
            var AvgScoreGreater80_1 = (from l in lst
                                       group l by new {l.Name, l.Term}
                                       into grouped
                                       where grouped.Average(m => m.Score) >= 80
                                       orderby grouped.Average(m => m.Score) descending
                                       select
                                           new
                                               {
                                                   grouped.Key.Name,
                                                   grouped.Key.Term,
                                                   Scores = grouped.Average(m => m.Score)
                                               }).ToList();
            foreach (var l in AvgScoreGreater80_1)
            {
                Console.WriteLine("姓名:{0},学期:{1},均分:{2}", l.Name, l.Term, l.Scores);
            }
            Console.WriteLine("---------第二种写法");
            /*此写法看起来较为复杂，第一个Groupby，由于是要对多个字段分组的，因此构建一个匿名对象，
            对这个匿名对象分组，分组得到的其实是一个IEnumberable<IGrouping<匿名类型，StudentScore>>这样一个类型。
            Where方法接受，和返回的都同样是IEnumberable<IGrouping<匿名类型，StudentScore>>类型，
            其中Where方法签名Func委托的类型也就成了Func<IGrouping<匿名类型，StudentScore>,bool>,
            之前说到，IGrouping<out TKey, out TElement>继承了IEnumerable<TElement>,
            因此这种类型可以有Average，Sum等方法。 */
            var AvgScoreGreater80_2 = lst.GroupBy(l => new {l.Name, l.Term})
                .Where(m => m.Average(x => x.Score) >= 80)
                .OrderByDescending(l => l.Average(x => x.Score))
                .Select(l => new {l.Key.Name, l.Key.Term, Scores = l.Average(m => m.Score)}).ToList();
            foreach (var l in AvgScoreGreater80_2)
            {
                Console.WriteLine("姓名:{0},学期:{1},均分:{2}", l.Name, l.Term, l.Scores);
            }
  
            Console.ReadKey();
        }

原文地址：http://hi.baidu.com/tewuapple/item/5e0e5a2862b67a8b9c63d103