CREATE TABLE teacher(
  t_id INT PRIMARY KEY AUTO_INCREMENT, 
  t_name VARCHAR()
 );
 CREATE TABLE class(
  c_id INT PRIMARY KEY AUTO_INCREMENT, 
  c_name VARCHAR(), 
  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('teacher');
 INSERT INTO teacher(t_name) VALUES('teacher');
 INSERT INTO class(c_name, teacher_id) VALUES('class_a', );
 INSERT INTO class(c_name, teacher_id) VALUES('class_b', 2); 

 package me.gacl.domain;
 /**
 * @author gacl
 * 定义teacher表对应的实体类
 */
 public class Teacher {
  //定义实体类的属性，与teacher表中的字段对应
  private int id;   //id===>t_id
  private String name; //name===>t_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 + "]";
  }
 } 

 package me.gacl.domain;
 /**
 * @author gacl
 * 定义class表对应的实体类
 */
 public class Classes {
  //定义实体类的属性，与class表中的字段对应
  private int id;   //id===>c_id
  private String name; //name===>c_name
  /**
  * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
  * 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的
  */
  private Teacher teacher;
  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;
  }
  @Override
  public String toString() {
   return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher+ "]";
  }
 } 

<?xml version="." encoding="UTF-" ?>
 <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper .//EN" "http://mybatis.org/dtd/mybatis--mapper.dtd">
 <!-- 为这个mapper指定一个唯一的namespace，namespace的值习惯上设置成包名+sql映射文件名，这样就能够保证namespace的值是唯一的
 例如namespace="me.gacl.mapping.classMapper"就是me.gacl.mapping(包名)+classMapper(classMapper.xml文件去除后缀)
 -->
 <mapper namespace="me.gacl.mapping.classMapper">
  <!-- 
   根据班级id查询班级信息(带老师的信息)
   ##. 联表查询
   SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=;
   ##. 执行两次查询
   SELECT * FROM class WHERE c_id=; //teacher_id=
   SELECT * FROM teacher WHERE t_id=;//使用上面得到的teacher_id
  -->
  <!-- 
  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 * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=
  -->
  <select id="getClass" parameterType="int" resultMap="ClassResultMap">
   select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
  </select>
  <!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
  <resultMap type="me.gacl.domain.Classes" id="ClassResultMap">
   <id property="id" column="c_id"/>
   <result property="name" column="c_name"/>
   <association property="teacher" javaType="me.gacl.domain.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=;
   SELECT * FROM teacher WHERE t_id= // 是上一个查询得到的teacher_id的值
  -->
  <select id="getClass" parameterType="int" resultMap="ClassResultMap">
   select * from class where c_id=#{id}
  </select>
  <!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
  <resultMap type="me.gacl.domain.Classes" id="ClassResultMap">
   <id property="id" column="c_id"/>
   <result property="name" column="c_name"/>
   <association property="teacher" column="teacher_id" select="getTeacher"/>
  </resultMap>
  <select id="getTeacher" parameterType="int" resultType="me.gacl.domain.Teacher">
   SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
  </select>
 </mapper> 

<mappers>
  <!-- 注册classMapper.xml文件， 
  classMapper.xml位于me.gacl.mapping这个包下，所以resource写成me/gacl/mapping/classMapper.xml-->
  <mapper resource="me/gacl/mapping/classMapper.xml"/>
</mappers> 

 package me.gacl.test;
 import me.gacl.domain.Classes;
 import me.gacl.util.MyBatisUtil;
 import org.apache.ibatis.session.SqlSession;
 import org.junit.Test;
 public class Test {
  @Test
  public void testGetClass(){
   SqlSession sqlSession = MyBatisUtil.getSqlSession();
   /**
   * 映射sql的标识字符串，
   * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
   * getClass是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL
   */
   String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的标识字符串
   //执行查询操作，将查询结果自动封装成Classes对象返回
   Classes clazz = sqlSession.selectOne(statement,);//查询class表中id为的记录
   //使用SqlSession执行完SQL之后需要关闭SqlSession
   sqlSession.close();
   System.out.println(clazz);//打印结果：Classes [id=, name=class_a, teacher=Teacher [id=, name=teacher]]
  }
  @Test
  public void testGetClass(){
   SqlSession sqlSession = MyBatisUtil.getSqlSession();
   /**
   * 映射sql的标识字符串，
   * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
   * getClass是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL
   */
   String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的标识字符串
   //执行查询操作，将查询结果自动封装成Classes对象返回
   Classes clazz = sqlSession.selectOne(statement,);//查询class表中id为的记录
   //使用SqlSession执行完SQL之后需要关闭SqlSession
   sqlSession.close();
   System.out.println(clazz);//打印结果：Classes [id=, name=class_a, teacher=Teacher [id=, name=teacher]]
  }
 } 

CREATE TABLE student(
 s_id INT PRIMARY KEY AUTO_INCREMENT, 
 s_name VARCHAR(20), 
 class_id INT
);
INSERT INTO student(s_name, class_id) VALUES('student_A', 1);
INSERT INTO student(s_name, class_id) VALUES('student_B', 1);
INSERT INTO student(s_name, class_id) VALUES('student_C', 1);
INSERT INTO student(s_name, class_id) VALUES('student_D', 2);
INSERT INTO student(s_name, class_id) VALUES('student_E', 2);
INSERT INTO student(s_name, class_id) VALUES('student_F', 2); 

 package me.gacl.domain;
 /**
 * @author gacl
 * 定义student表所对应的实体类
 */
 public class Student {
  //定义属性，和student表中的字段对应
  private int id;   //id===>s_id
  private String name; //name===>s_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 "Student [id=" + id + ", name=" + name + "]";
  }
 } 

 package me.gacl.domain;
 import java.util.List;
 /**
 * @author gacl
 * 定义class表对应的实体类
 */
 public class Classes {
  //定义实体类的属性，与class表中的字段对应
  private int id;   //id===>c_id
  private String name; //name===>c_name
  /**
  * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
  * 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的
  */
  private Teacher teacher;
  //使用一个List<Student>集合属性表示班级拥有的学生
  private List<Student> students;
  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;
  }
  public List<Student> getStudents() {
   return students;
  }
  public void setStudents(List<Student> students) {
   this.students = students;
  }
  @Override
  public String toString() {
   return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher
     + ", students=" + students + "]";
  }
 } 

<!-- 
   根据classId查询对应的班级信息,包括学生,老师
  -->
  <!-- 
  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
  SELECT * FROM class c, teacher t,student s WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND c.c_id=
  -->
  <select id="getClass" parameterType="int" resultMap="ClassResultMap">
   select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and c.c_id=#{id}
  </select>
  <resultMap type="me.gacl.domain.Classes" id="ClassResultMap">
   <id property="id" column="c_id"/>
   <result property="name" column="c_name"/>
   <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher">
    <id property="id" column="t_id"/>
    <result property="name" column="t_name"/>
   </association>
   <!-- ofType指定students集合中的对象类型 -->
   <collection property="students" ofType="me.gacl.domain.Student">
    <id property="id" column="s_id"/>
    <result property="name" column="s_name"/>
   </collection>
  </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=;
    SELECT * FROM teacher WHERE t_id= // 是上一个查询得到的teacher_id的值
    SELECT * FROM student WHERE class_id= //是第一个查询得到的c_id字段的值
  -->
  <select id="getClass" parameterType="int" resultMap="ClassResultMap">
   select * from class where c_id=#{id}
  </select>
  <resultMap type="me.gacl.domain.Classes" id="ClassResultMap">
   <id property="id" column="c_id"/>
   <result property="name" column="c_name"/>
   <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher" select="getTeacher"></association>
   <collection property="students" ofType="me.gacl.domain.Student" column="c_id" select="getStudent"></collection>
  </resultMap>
  <select id="getTeacher" parameterType="int" resultType="me.gacl.domain.Teacher">
   SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
  </select>
  <select id="getStudent" parameterType="int" resultType="me.gacl.domain.Student">
   SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
  </select> 

package me.gacl.test;
 import me.gacl.domain.Classes;
 import me.gacl.util.MyBatisUtil;
 import org.apache.ibatis.session.SqlSession;
 import org.junit.Test;
 public class Test {
  @Test
  public void testGetClass(){
   SqlSession sqlSession = MyBatisUtil.getSqlSession();
   /**
   * 映射sql的标识字符串，
   * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
   * getClass是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL
   */
   String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的标识字符串
   //执行查询操作，将查询结果自动封装成Classes对象返回
   Classes clazz = sqlSession.selectOne(statement,);//查询class表中id为的记录
   //使用SqlSession执行完SQL之后需要关闭SqlSession
   sqlSession.close();
   //打印结果：Classes [id=, name=class_a, teacher=Teacher [id=, name=teacher], students=[Student [id=, name=student_A], Student [id=, name=student_B], Student [id=, name=student_C]]]
   System.out.println(clazz);
  }
  @Test
  public void testGetClass(){
   SqlSession sqlSession = MyBatisUtil.getSqlSession();
   /**
   * 映射sql的标识字符串，
   * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
   * getClass是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL
   */
   String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的标识字符串
   //执行查询操作，将查询结果自动封装成Classes对象返回
   Classes clazz = sqlSession.selectOne(statement,);//查询class表中id为的记录
   //使用SqlSession执行完SQL之后需要关闭SqlSession
   sqlSession.close();
   //打印结果：Classes [id=, name=class_a, teacher=Teacher [id=, name=teacher], students=[Student [id=, name=student_A], Student [id=, name=student_B], Student [id=, name=student_C]]]
   System.out.println(clazz);
  }
 }