LINQ操作符二：SelectMany

SelectMany操作符提供了将多个from子句组合起来的功能，相当于数据库中的多表连接查询，它将每个对象的结果合并成单个序列。

示例：

student类：

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;

 namespace SelectMany操作符
 {
     /// <summary>
     /// 学生类
     /// </summary>
     public class Student
     {
         //姓名
         public string Name { get; set; }
         //成绩
         public int Score { get; set; }
         //构造函数
         public Student(string name, int score)
         {
             this.Name = name;
             this.Score = score;
         }
     }
 }

teacher类：

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;

 namespace SelectMany操作符
 {
     /// <summary>
     /// Teacher类
     /// </summary>
     public class Teacher
     {
         //姓名
         public string Name { get; set; }
         //学生集合
         public List<Student> Students { get; set; }

         public Teacher(string name, List<Student> students)
         {
             this.Name = name;
             this.Students = students;
         }
     }
 }

Program类

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;

 namespace SelectMany操作符
 {
     class Program
     {
         static void Main(string[] args)
         {
             //使用集合初始化器初始化Teacher集合
             List<Teacher> teachers = new List<Teacher> {
                new Teacher("徐老师",
                new List<Student>(){
                  new Student("宋江",),
                 new Student("卢俊义",),
                 new Student("朱武",)
                }
                ),
                 new Teacher("姜老师",
                new List<Student>(){
                  new Student("林冲",),
                 new Student("花荣",),
                 new Student("柴进",)
                }
                ),
                 new Teacher("樊老师",
                new List<Student>(){
                  new Student("关胜",),
                 new Student("阮小七",),
                 new Student("时迁",)
                }
                )
             };

             //问题：查询Score小于60的学生
             //方法1：循环遍历、会有性能的损失
             foreach (Teacher t in teachers)
             {
                 foreach (Student s in t.Students)
                 {
                     if (s.Score < )
                     {
                         Console.WriteLine("姓名:" + s.Name + ",成绩:"+s.Score);
                     }
                 }
             }

             //查询表达式
             //方法2：使用SelectMany  延迟加载：在不需要数据的时候，就不执行调用数据，能减轻程序和数据库的交互，可以提供程序的性能，执行循环的时候才去访问数据库取数据
             //直接返回学生的数据
             var query = from t in teachers
                         from s in t.Students
                         where s.Score <
                         select s;
             foreach (var item in query)
             {
                 Console.WriteLine("姓名:" + item.Name + ",成绩:"+item.Score);
             }
             //只返回老师的数据
             var query1 = from t in teachers
                          from s in t.Students
                          where s.Score <
                          select new {
                             t,
                             teacherName=t.Name,
                             student=t.Students.Where(p=>p.Score<).ToList()
                          };
             foreach (var item in query1)
             {
                 Console.WriteLine("老师姓名:" + item.teacherName + ",学生姓名:" +item.student.FirstOrDefault().Name+ ",成绩:" + item.student.FirstOrDefault().Score);
             }
             // 使用匿名类 返回老师和学生的数据
             var query2 = from t in teachers
                          from s in t.Students
                          where s.Score <
                          select new { teacherName=t.Name, studentName=s.Name,studentScore=s.Score };
             foreach (var item in query2)
             {
                 Console.WriteLine("老师姓名:" + item.teacherName + ",学生姓名:" + item.studentName + ",成绩:" + item.studentScore);
             }

             //使用查询方法
             var query3 = teachers.SelectMany(p => p.Students.Where(t=>t.Score<).ToList());
             foreach (var item in query3)
             {
                 Console.WriteLine("姓名:" + item.Name + ",成绩:" + item.Score);
             }
             Console.ReadKey();

         }
     }
 }