1 CREATE TABLE teacher( 2     t_id INT PRIMARY KEY AUTO_INCREMENT,  3     t_name VARCHAR(20) 4 ); 5 CREATE TABLE class( 6     c_id INT PRIMARY KEY AUTO_INCREMENT,  7     c_name VARCHAR(20),  8     teacher_id INT 9 );10 ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);    11 12 INSERT INTO teacher(t_name) VALUES(‘teacher1‘);13 INSERT INTO teacher(t_name) VALUES(‘teacher2‘);14 15 INSERT INTO class(c_name, teacher_id) VALUES(‘class_a‘, 1);16 INSERT INTO class(c_name, teacher_id) VALUES(‘class_b‘, 2);

 1 package me.gacl.domain; 2  3 /** 4  * @author gacl 5  * 定义teacher表对应的实体类 6  */ 7 public class Teacher { 8  9     //定义实体类的属性，与teacher表中的字段对应10     private int id;            //id===>t_id11     private String name;    //name===>t_name12 13     public int getId() {14         return id;15     }16 17     public void setId(int id) {18         this.id = id;19     }20 21     public String getName() {22         return name;23     }24 25     public void setName(String name) {26         this.name = name;27     }28 29     @Override30     public String toString() {31         return "Teacher [id=" + id + ", name=" + name + "]";32     }33 }

 1 package me.gacl.domain; 2  3 /** 4  * @author gacl 5  * 定义class表对应的实体类 6  */ 7 public class Classes { 8  9     //定义实体类的属性，与class表中的字段对应10     private int id;            //id===>c_id11     private String name;    //name===>c_name12     13     /**14      * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，15      * 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16      */17     private Teacher teacher;18 19     public int getId() {20         return id;21     }22 23     public void setId(int id) {24         this.id = id;25     }26 27     public String getName() {28         return name;29     }30 31     public void setName(String name) {32         this.name = name;33     }34 35     public Teacher getTeacher() {36         return teacher;37     }38 39     public void setTeacher(Teacher teacher) {40         this.teacher = teacher;41     }42 43     @Override44     public String toString() {45         return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher+ "]";46     }47 }

 1 <?xml version="1.0" encoding="UTF-8" ?> 2 <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> 3 <!-- 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 4 例如namespace="me.gacl.mapping.classMapper"就是me.gacl.mapping(包名)+classMapper(classMapper.xml文件去除后缀) 5  --> 6 <mapper namespace="me.gacl.mapping.classMapper"> 7  8     <!--  9         根据班级id查询班级信息(带老师的信息)10         ##1. 联表查询11         SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1;12         13         ##2. 执行两次查询14         SELECT * FROM class WHERE c_id=1;  //teacher_id=115         SELECT * FROM teacher WHERE t_id=1;//使用上面得到的teacher_id16      -->17 18     <!-- 19     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20              封装联表查询的数据(去除重复的数据)21         select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=122     -->23     <select id="getClass" parameterType="int" resultMap="ClassResultMap">24         select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}25     </select>26     <!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->27     <resultMap type="me.gacl.domain.Classes" id="ClassResultMap">28         <id property="id" column="c_id"/>29         <result property="name" column="c_name"/>30         <association property="teacher" javaType="me.gacl.domain.Teacher">31             <id property="id" column="t_id"/>32             <result property="name" column="t_name"/>33         </association>34     </resultMap>35     36     <!-- 37     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38         SELECT * FROM class WHERE c_id=1;39         SELECT * FROM teacher WHERE t_id=1   //1 是上一个查询得到的teacher_id的值40     -->41      <select id="getClass2" parameterType="int" resultMap="ClassResultMap2">42         select * from class where c_id=#{id}43      </select>44      <!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->45      <resultMap type="me.gacl.domain.Classes" id="ClassResultMap2">46         <id property="id" column="c_id"/>47         <result property="name" column="c_name"/>48         <association property="teacher" column="teacher_id" select="getTeacher"/>49      </resultMap>50      51      <select id="getTeacher" parameterType="int" resultType="me.gacl.domain.Teacher">52         SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}53      </select>54 55 </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>

 1 package me.gacl.test; 2  3 import me.gacl.domain.Classes; 4 import me.gacl.util.MyBatisUtil; 5 import org.apache.ibatis.session.SqlSession; 6 import org.junit.Test; 7  8 public class Test3 { 9     10     @Test11     public void testGetClass(){12         SqlSession sqlSession = MyBatisUtil.getSqlSession();13         /**14          * 映射sql的标识字符串，15          * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，16          * 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SQL17          */18         String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的标识字符串19         //执行查询操作，将查询结果自动封装成Classes对象返回20         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录21         //使用SqlSession执行完SQL之后需要关闭SqlSession22         sqlSession.close();23         System.out.println(clazz);//打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]24     }25     26     @Test27     public void testGetClass2(){28         SqlSession sqlSession = MyBatisUtil.getSqlSession();29         /**30          * 映射sql的标识字符串，31          * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，32          * getClass2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SQL33          */34         String statement = "me.gacl.mapping.classMapper.getClass2";//映射sql的标识字符串35         //执行查询操作，将查询结果自动封装成Classes对象返回36         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录37         //使用SqlSession执行完SQL之后需要关闭SqlSession38         sqlSession.close();39         System.out.println(clazz);//打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]40     }41 }

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);

 1 package me.gacl.domain; 2  3 /** 4  * @author gacl 5  * 定义student表所对应的实体类 6  */ 7 public class Student { 8  9     //定义属性，和student表中的字段对应10     private int id;            //id===>s_id11     private String name;    //name===>s_name12     13     public int getId() {14         return id;15     }16 17     public void setId(int id) {18         this.id = id;19     }20 21     public String getName() {22         return name;23     }24 25     public void setName(String name) {26         this.name = name;27     }28 29     @Override30     public String toString() {31         return "Student [id=" + id + ", name=" + name + "]";32     }33 }

 1 package me.gacl.domain; 2  3 import java.util.List; 4  5 /** 6  * @author gacl 7  * 定义class表对应的实体类 8  */ 9 public class Classes {10 11     //定义实体类的属性，与class表中的字段对应12     private int id;            //id===>c_id13     private String name;    //name===>c_name14     15     /**16      * class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，17      * 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18      */19     private Teacher teacher;20     //使用一个List<Student>集合属性表示班级拥有的学生21     private List<Student> students;22 23     public int getId() {24         return id;25     }26 27     public void setId(int id) {28         this.id = id;29     }30 31     public String getName() {32         return name;33     }34 35     public void setName(String name) {36         this.name = name;37     }38 39     public Teacher getTeacher() {40         return teacher;41     }42 43     public void setTeacher(Teacher teacher) {44         this.teacher = teacher;45     }46 47     public List<Student> getStudents() {48         return students;49     }50 51     public void setStudents(List<Student> students) {52         this.students = students;53     }54 55     @Override56     public String toString() {57         return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher58                 + ", students=" + students + "]";59     }60 }

 1 <!--  2         根据classId查询对应的班级信息,包括学生,老师 3      --> 4     <!--  5     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 6     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=1 7      --> 8     <select id="getClass3" parameterType="int" resultMap="ClassResultMap3"> 9         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}10     </select>11     <resultMap type="me.gacl.domain.Classes" id="ClassResultMap3">12         <id property="id" column="c_id"/>13         <result property="name" column="c_name"/>14         <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher">15             <id property="id" column="t_id"/>16             <result property="name" column="t_name"/>17         </association>18         <!-- ofType指定students集合中的对象类型 -->19         <collection property="students" ofType="me.gacl.domain.Student">20             <id property="id" column="s_id"/>21             <result property="name" column="s_name"/>22         </collection>23     </resultMap>24     25     <!-- 26         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27             SELECT * FROM class WHERE c_id=1;28             SELECT * FROM teacher WHERE t_id=1   //1 是上一个查询得到的teacher_id的值29             SELECT * FROM student WHERE class_id=1  //1是第一个查询得到的c_id字段的值30      -->31      <select id="getClass4" parameterType="int" resultMap="ClassResultMap4">32         select * from class where c_id=#{id}33      </select>34      <resultMap type="me.gacl.domain.Classes" id="ClassResultMap4">35         <id property="id" column="c_id"/>36         <result property="name" column="c_name"/>37         <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher" select="getTeacher2"></association>38         <collection property="students" ofType="me.gacl.domain.Student" column="c_id" select="getStudent"></collection>39      </resultMap>40      41      <select id="getTeacher2" parameterType="int" resultType="me.gacl.domain.Teacher">42         SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}43      </select>44      45      <select id="getStudent" parameterType="int" resultType="me.gacl.domain.Student">46         SELECT s_id id, s_name name FROM student WHERE class_id=#{id}47      </select>

 1 package me.gacl.test; 2  3 import me.gacl.domain.Classes; 4 import me.gacl.util.MyBatisUtil; 5 import org.apache.ibatis.session.SqlSession; 6 import org.junit.Test; 7  8 public class Test4 { 9     10     @Test11     public void testGetClass3(){12         SqlSession sqlSession = MyBatisUtil.getSqlSession();13         /**14          * 映射sql的标识字符串，15          * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，16          * getClass3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SQL17          */18         String statement = "me.gacl.mapping.classMapper.getClass3";//映射sql的标识字符串19         //执行查询操作，将查询结果自动封装成Classes对象返回20         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录21         //使用SqlSession执行完SQL之后需要关闭SqlSession22         sqlSession.close();23         //打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]24         System.out.println(clazz);25     }26     27     @Test28     public void testGetClass4(){29         SqlSession sqlSession = MyBatisUtil.getSqlSession();30         /**31          * 映射sql的标识字符串，32          * me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，33          * getClass4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SQL34          */35         String statement = "me.gacl.mapping.classMapper.getClass4";//映射sql的标识字符串36         //执行查询操作，将查询结果自动封装成Classes对象返回37         Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录38         //使用SqlSession执行完SQL之后需要关闭SqlSession39         sqlSession.close();40         //打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]41         System.out.println(clazz);42     }43 }