using  System;
using  System.Collections.Generic;
using  System.Text;

namespace  BLL
{

     ///   <summary>
     ///  封装一个包含2张表字段的实体
     ///   </summary>
     public   class  StudentAndScore
    {

         int  code;
         public   int  Code
        {

             get  {  return  code; }
             set  { code  =  value; }
        }

         string  name;
         public   string  Name
        {

             get  {  return  name; }
             set  { name  =  value; }
        }

         float ?  stuScore;
         public   float ?  StuScore
        {

             get  {  return  stuScore; }
             set  { stuScore  =  value; }
        }
    }
}

using  System;
using  System.Collections.Generic;
using  System.Text;
// myself
using  DAL;
using  Entity;

namespace  BLL
{

     ///   <summary>
     ///  获得2张表对应的数据
     ///   </summary>
     public   class  StudentAndTeacherBLL
    {

         ///   <summary>
         ///  注意：该方法获得所有学生的成绩，
         ///        属于“左外联接”(当没有成绩时，成绩显示为空)。
         ///        若要实现“内联接”的显示效果，
         ///        则去掉：if(stuScores.Count == 0) 的判断即可！
         ///   </summary>
         ///   <returns> 2张表数据的结果集 </returns>
         public  List < StudentAndScore >  GetStudentAndTeacher()
        {

             // 用来保存结果集
            List < StudentAndScore >  allStudentAndScore  =   new  List < StudentAndScore > ();
             // 获得所有学生的信息
            List < Student >  students  =   new  StudentDAL().GetAllStudent();
             foreach  (Student stu  in  students)
            {

                 // 根据学号获得该学生的所有成绩
                List < StuScore >  stuScores  =   new  ScoreDAL().GetAllScore(stu.Code);

                 if  (stuScores.Count  ==   0 )  // 如果没有该学生的成绩
                {

                     // 用来保存单个联接实体
                    StudentAndScore studentAndScore  =   new  StudentAndScore();
                    studentAndScore.Code  =  stu.Code;
                    studentAndScore.Name  =  stu.Name;
                    studentAndScore.StuScore  =   null ;  // 成绩为null
                     // 添加到结果集
                    allStudentAndScore.Add(studentAndScore);
                }
                 else   // 如果有该学生的成绩
                {

                     foreach  (StuScore SScore  in  stuScores)
                    {

                         // 用来保存单个联接实体
                        StudentAndScore studentAndScore  =   new  StudentAndScore();
                        studentAndScore.Code  =  stu.Code;
                        studentAndScore.Name  =  stu.Name;
                        studentAndScore.StuScore  =  SScore.Score;
                         // 添加到结果集
                        allStudentAndScore.Add(studentAndScore);
                    }
                }
            }
             return  allStudentAndScore;  // 返回结果集
        }
    }
}