表的設計和Mybatis多對一相同

表的設計
利用Navicat for mysql建表，然後在檢視選單點選ER圖表（概念模型） 如下：

設計好的表ER圖表

上述表 student tid設定外來鍵參考teacher的id，所以建好表後兩者都為空要先新增teacher的資料。

多個學生對於一個老師
實體類
Teacher

public class Teacher {
    private int id;
    private String name;
    private List<Student>  students;
    //省略set/get方法
 

}

Student

public class Student {
    private int id;
    private String name;
    //省略set/get方法
}

編寫teacher.mapper.xml對映檔案

兩種處理方式
第一種 結果巢狀

<mapper namespace="cn.sxt.entity.teacher.mapper">
    <select id="getTeacher" resultMap="TeacherStudent">
        select  s.id sid,s.name sname,s.tid stid ,t.id
 
 tid,t.name tname from student s,teacher t where s.tid=t.id and tid=#{id}
    </select>
    <resultMap  type="Teacher"  id="TeacherStudent">
        <id column="tid"  property="id"></id>
        <result column="tname" property="name"></result>
        <collection  property
 
="students"  javaType="ArrayList"   ofType="Student">
            <id column="sid"  property="id"></id>
            <result column="sname"  property="name"></result>
        </collection>
    </resultMap>
</mapper>

第二種 查詢巢狀
teacher.mapper.xml

<select id="getTeacher"  resultMap="TeacherStudent">
        select * from teacher where id=#{id}
    </select>
    <resultMap id="TeacherStudent"  type="Teacher">
        <collection  property="students"  ofType="Student"  column="id"  select="cn.sxt.entity.student.mapper.getStudentById"></collection>
    </resultMap>

student.mapper.xml

<mapper namespace="cn.sxt.entity.student.mapper">
    <select id="getStudentById"    resultType="Student">
        select * from  student where tid=#{id}
    </select>
</mapper>

使用

public static void main(String[] args) throws IOException {
        TeacherDao teacherDao=new TeacherDao();
        Teacher  teacher=teacherDao.getTeacher(1);
        System.out.println("teacher name="+teacher.getName());
        List<Student>  list=teacher.getStudents();
        for(Student stu:list){
            System.out.println("student name="+stu.getName());
        }
    }