static void Main(string[] args)
        {
            List<Person> persons = new List<Person>();

            persons.Add(, ClassId = ", Name = "小明" });
            persons.Add(, ClassId = ", Name = "小明2" });
            persons.Add(, ClassId = ", Name = "小明3" });
            persons.Add(, ClassId = ", Name = "小明4" });
            persons.Add(, ClassId = ", Name = "小明5" });
            persons.Add(, ClassId = ", Name = "小明6" });
            List<Classes> classes = new List<Classes>();
            classes.Add(", ClassName = "1年级" });
            classes.Add(", ClassName = "2年级" });
            classes.Add(", ClassName = "3年级" });
            classes.Add(", ClassName = "4年级" });
            classes.Add(", ClassName = "5年级" });
            //查询年纪大于12 小于17的学生
            List<Person> person = ( && p.Age <  select p).ToList<Person>();

            //表连接
            var datas = from p in persons
                        join c in classes on p.ClassId equals c.ClassId
                        select new { p.Name, p.Age, c.ClassName };
            //查询每个班级每个年龄段的人数 并排序
            var data = from c in classes
                       join p in persons on c.ClassId equals p.ClassId

                       group c by new {c.ClassName,p.Age} into g
                       orderby g.Count() descending
                       select new { g.Key,Count = g.Count() }
                       ;
            Console.ReadKey();
        }
    }