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",80),
                new Student("卢俊义",95),
                new Student("朱武",45)
               }
               ),
                new Teacher("姜老师",
               new List<Student>(){
                 new Student("林冲",90),
                new Student("花荣",85),
                new Student("柴进",58)
               }
               ),
                new Teacher("樊老师",
               new List<Student>(){
                 new Student("关胜",100),
                new Student("阮小七",70),
                new Student("时迁",30)
               }
               )
            };

            //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 < 60)
                    {
                        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 < 60
                        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 < 60
                         select new { 
                            t,
                            teacherName=t.Name,
                            student=t.Students.Where(p=>p.Score<60).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 < 60
                         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<60).ToList());
            foreach (var item in query3)
            {
                Console.WriteLine("姓名:" + item.Name + ",成绩:" + item.Score);
            }
            Console.ReadKey();

        }
    }
}