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：

 1 using System;
 2 using System.Collections.Generic;
 3 using System.Linq;
 4 using System.Text;
 5 using System.Threading.Tasks;
 6
 7 namespace SelectMany操作符
 8 {
 9     class Program
10     {
11         static void Main(string[] args)
12         {
13             //使用集合初始化器初始化Teacher集合
14             List<Teacher> teachers = new List<Teacher> {
15                new Teacher("徐老师",
16                new List<Student>(){
17                  new Student("宋江",80),
18                 new Student("卢俊义",95),
19                 new Student("朱武",45)
20                }
21                ),
22                 new Teacher("姜老师",
23                new List<Student>(){
24                  new Student("林冲",90),
25                 new Student("花荣",85),
26                 new Student("柴进",58)
27                }
28                ),
29                 new Teacher("樊老师",
30                new List<Student>(){
31                  new Student("关胜",100),
32                 new Student("阮小七",70),
33                 new Student("时迁",30)
34                }
35                )
36             };
37
38             //问题：查询Score小于60的学生
39             //方法1：循环遍历、会有性能的损失
40             foreach (Teacher t in teachers)
41             {
42                 foreach (Student s in t.Students)
43                 {
44                     if (s.Score < 60)
45                     {
46                         Console.WriteLine("姓名:" + s.Name + ",成绩:"+s.Score);
47                     }
48                 }
49             }
50
51             //查询表达式
52             //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
53             //直接返回学生的数据
54             var query = from t in teachers
55                         from s in t.Students
56                         where s.Score < 60
57                         select s;
58             foreach (var item in query)
59             {
60                 Console.WriteLine("姓名:" + item.Name + ",成绩:"+item.Score);
61             }
62             //只返回老师的数据
63             var query1 = from t in teachers
64                          from s in t.Students
65                          where s.Score < 60
66                          select new {
67                             t,
68                             teacherName=t.Name,
69                             student=t.Students.Where(p=>p.Score<60).ToList()
70                          };
71             foreach (var item in query1)
72             {
73                 Console.WriteLine("老师姓名:" + item.teacherName + ",学生姓名:" +item.student.FirstOrDefault().Name+ ",成绩:" + item.student.FirstOrDefault().Score);
74             }
75             // 使用匿名类 返回老师和学生的数据
76             var query2 = from t in teachers
77                          from s in t.Students
78                          where s.Score < 60
79                          select new { teacherName=t.Name, studentName=s.Name,studentScore=s.Score };
80             foreach (var item in query2)
81             {
82                 Console.WriteLine("老师姓名:" + item.teacherName + ",学生姓名:" + item.studentName + ",成绩:" + item.studentScore);
83             }
84
85             //使用查询方法
86             var query3 = teachers.SelectMany(p => p.Students.Where(t=>t.Score<60).ToList());
87             foreach (var item in query3)
88             {
89                 Console.WriteLine("姓名:" + item.Name + ",成绩:" + item.Score);
90             }
91             Console.ReadKey();
92
93         }
94     }
95 }