CREATE TABLE teacher(

 t_id INT PRIMARY KEY AUTO_INCREMENT, t_name VARCHAR(20)

 );

 CREATE TABLE class(

 c_id INT PRIMARY KEY AUTO_INCREMENT, c_name VARCHAR(20),

 teacher_id INT

 );

 ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);

 INSERT INTO teacher(t_name) VALUES('LS1'); INSERT INTO teacher(t_name) VALUES('LS2');

 INSERT INTO class(c_name, teacher_id) VALUES('bj_a', 1); INSERT INTO class(c_name, teacher_id) VALUES('bj_b', 2);

 package com.zhengbin.entity;

 public class Teacher {

     private int id;

     private String name;

     public int getId() {

         return id;

     }

     public void setId(int id) {

         this.id = id;

     }

     public String getName() {

         return name;

     }

     public void setName(String name) {

         this.name = name;

     }

     @Override

     public String toString() {

         return "Teacher [id=" + id + ", name=" + name + "]";

     }

     public Teacher(int id, String name) {

         super();

         this.id = id;

         this.name = name;

     }

     public Teacher() {

         super();

         // TODO Auto-generated constructor stub

     }

 }

 package com.zhengbin.entity;

 public class Classes {

     private int id;

     private String name;

     private Teacher teacher;

     @Override

     public String toString() {

         return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher + "]";

     }

     public Classes() {

         super();

     }

     public int getId() {

         return id;

     }

     public void setId(int id) {

         this.id = id;

     }

     public String getName() {

         return name;

     }

     public void setName(String name) {

         this.name = name;

     }

     public Teacher getTeacher() {

         return teacher;

     }

     public void setTeacher(Teacher teacher) {

         this.teacher = teacher;

     }

 }

 <?xml version="1.0" encoding="UTF-8" ?>

 <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

 <!-- 每个映射文件的namespace应该是唯一的 -->

 <mapper namespace="com.zhengbin.entity.classMapper">

     <!-- parameterType 参数表示需要参数的类型 -->

     <!-- resultType 参数表示返回结果的类型，该可以写为实体包的全路径，或者在conf.xml配置文件中，声明实体的别名 -->

     <!--

         方式一：

             嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集 封装联表查询的数据(去除重复的数据)

      -->

     <select id="getClass" parameterType="int" resultMap="getClassMap">

         select * from class c,teacher t where c.teacher_id=t.t_id and c.c_id=#{id};

     </select>

     <resultMap type="Classes" id="getClassMap">

         <id property="id" column="c_id"/>

         <result property="name" column="c_name"/>

         <association property="teacher" javaType="Teacher">

             <id property="id" column="t_id"/>

             <result property="name" column="t_name"/>

         </association>

     </resultMap>

     <!--

         方式二：

             嵌套查询：通过执行另外一个 SQL 映射语句来返回预期的复杂类型

                     SELECT * FROM class WHERE c_id=1;

                     SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的 teacher_id 的值

      -->

      <select id="getClass2" parameterType="int" resultMap="getClass2Map">

          select * from class where c_id=#{id}

      </select>

      <select id="getTeacher" parameterType="int" resultType="Teacher">

          select t_id id,t_name name from teacher where t_id=#{id}

      </select>

      <resultMap type="Classes" id="getClass2Map">

          <id property="id" column="c_id"/>

          <result property="name" column="c_name"/>

          <association property="teacher" select="getTeacher" column="teacher_id"/>

      </resultMap>

 </mapper>

 package com.zhengbin.test;

 import org.apache.ibatis.session.SqlSession;

 import org.apache.ibatis.session.SqlSessionFactory;

 import com.zhengbin.entity.Classes;

 import com.zhengbin.util.MyBatisUtils;

 public class Test3 {

     @org.junit.Test

     public void getTeacher(){

         SqlSessionFactory sessionFactory = MyBatisUtils.getFactory();

         // 参数为TRUE，相当于session.commit();

         SqlSession session = sessionFactory.openSession(true);

         // 读取映射文件

         String statement = "com.zhengbin.entity.classMapper" + ".getClass";

 //        String statement = "com.zhengbin.entity.classMapper" + ".getClass2";

         Classes c = session.selectOne(statement,1);

         System.out.println(c);

         session.close();

     }

 }