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 * 定義teacher表對應的實體類 */ public class Teacher {  //定義實體類的屬性，與teacher表中的字段對應  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 * 定義class表對應的實體類 */ public class Classes {  //定義實體類的屬性，與class表中的字段對應  private int id;   //id===>c_id  private String name; //name===>c_name  /**  * class表中有一個teacher_id字段，所以在Classes類中定義一個teacher屬性，  * 用于維護teacher和class之間的一對一關系，通過這個teacher屬性就可以知道這個班級是由哪個老師負責的  */  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"> <!-- 為這個mapper指定一個唯一的namespace，namespace的值習慣上設置成包名+sql映射文件名，這樣就能夠保證namespace的值是唯一的 例如namespace="me.gacl.mapping.classMapper"就是me.gacl.mapping(包名)+classMapper(classMapper.xml文件去除后綴) --> <mapper namespace="me.gacl.mapping.classMapper">  <!--    根據班級id查詢班級信息(帶老師的信息)   ##. 聯表查詢   SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=;   ##. 執行兩次查詢   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：嵌套結果：使用嵌套結果映射來處理重復的聯合結果的子集    封裝聯表查詢的數據(去除重復的數據)   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映射實體類和字段之間的一一對應關系 -->  <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：嵌套查詢：通過執行另外一個SQL映射語句來返回預期的復雜類型   SELECT * FROM class WHERE c_id=;   SELECT * FROM teacher WHERE t_id= // 是上一個查詢得到的teacher_id的值  -->  <select id="getClass" parameterType="int" resultMap="ClassResultMap">   select * from class where c_id=#{id}  </select>  <!-- 使用resultMap映射實體類和字段之間的一一對應關系 -->  <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>  <!-- 注冊classMapper.xml文件，   classMapper.xml位于me.gacl.mapping這個包下，所以resource寫成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的標識字符串，   * me.gacl.mapping.classMapper是classMapper.xml文件中mapper標簽的namespace屬性的值，   * getClass是select標簽的id屬性值，通過select標簽的id屬性值就可以找到要執行的SQL   */   String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的標識字符串   //執行查詢操作，將查詢結果自動封裝成Classes對象返回   Classes clazz = sqlSession.selectOne(statement,);//查詢class表中id為的記錄   //使用SqlSession執行完SQL之后需要關閉SqlSession   sqlSession.close();   System.out.println(clazz);//打印結果：Classes [id=, name=class_a, teacher=Teacher [id=, name=teacher]]  }  @Test  public void testGetClass(){   SqlSession sqlSession = MyBatisUtil.getSqlSession();   /**   * 映射sql的標識字符串，   * me.gacl.mapping.classMapper是classMapper.xml文件中mapper標簽的namespace屬性的值，   * getClass是select標簽的id屬性值，通過select標簽的id屬性值就可以找到要執行的SQL   */   String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的標識字符串   //執行查詢操作，將查詢結果自動封裝成Classes對象返回   Classes clazz = sqlSession.selectOne(statement,);//查詢class表中id為的記錄   //使用SqlSession執行完SQL之后需要關閉SqlSession   sqlSession.close();   System.out.println(clazz);//打印結果：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 * 定義student表所對應的實體類 */ public class Student {  //定義屬性，和student表中的字段對應  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 * 定義class表對應的實體類 */ public class Classes {  //定義實體類的屬性，與class表中的字段對應  private int id;   //id===>c_id  private String name; //name===>c_name  /**  * class表中有一個teacher_id字段，所以在Classes類中定義一個teacher屬性，  * 用于維護teacher和class之間的一對一關系，通過這個teacher屬性就可以知道這個班級是由哪個老師負責的  */  private Teacher teacher;  //使用一個List<Student>集合屬性表示班級擁有的學生  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 + "]";  } } 

<!--    根據classId查詢對應的班級信息,包括學生,老師  -->  <!--   方式一: 嵌套結果: 使用嵌套結果映射來處理重復的聯合結果的子集  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集合中的對象類型 -->   <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：嵌套查詢：通過執行另外一個SQL映射語句來返回預期的復雜類型    SELECT * FROM class WHERE c_id=;    SELECT * FROM teacher WHERE t_id= // 是上一個查詢得到的teacher_id的值    SELECT * FROM student WHERE class_id= //是第一個查詢得到的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的標識字符串，   * me.gacl.mapping.classMapper是classMapper.xml文件中mapper標簽的namespace屬性的值，   * getClass是select標簽的id屬性值，通過select標簽的id屬性值就可以找到要執行的SQL   */   String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的標識字符串   //執行查詢操作，將查詢結果自動封裝成Classes對象返回   Classes clazz = sqlSession.selectOne(statement,);//查詢class表中id為的記錄   //使用SqlSession執行完SQL之后需要關閉SqlSession   sqlSession.close();   //打印結果：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的標識字符串，   * me.gacl.mapping.classMapper是classMapper.xml文件中mapper標簽的namespace屬性的值，   * getClass是select標簽的id屬性值，通過select標簽的id屬性值就可以找到要執行的SQL   */   String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的標識字符串   //執行查詢操作，將查詢結果自動封裝成Classes對象返回   Classes clazz = sqlSession.selectOne(statement,);//查詢class表中id為的記錄   //使用SqlSession執行完SQL之后需要關閉SqlSession   sqlSession.close();   //打印結果：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);  } }