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假设每个学生都有一名指导老师，本示例的任务就是查询出学生的详细信息，这就包括学生的指导老师的信息。具体代码如下：

1、表的结构

DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (`id` int(11) NOT NULL auto_increment,`name` varchar(10) NOT NULL,`gender` char(1) NOT NULL,`major` varchar(20) NOT NULL,`grade` char(4) NOT NULL,`supervisor_id` int(11) NOT NULL,PRIMARY KEY  (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', '李林', '男', '计算机科学与技术', '2011', '1');
-- ----------------------------
-- Table structure for teacher
-- ----------------------------
DROP TABLE IF EXISTS `teacher`;
CREATE TABLE `teacher` (`id` int(11) NOT NULL auto_increment,`name` varchar(10) NOT NULL,`gender` char(1) NOT NULL,`research_area` varchar(20) NOT NULL,PRIMARY KEY  (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of teacher
-- ----------------------------
INSERT INTO `teacher` VALUES ('1', '张伟', '男', '软件工程');

 2、对应的JavaBean类

public class Student {private int id;  private String name;     // 姓名private String gender;   // 性别private String major;    // 专业private String grade;    // 年级private Teacher supervisor; //指导教师 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 String getGender() {return gender;}public void setGender(String gender) {this.gender = gender;}public String getMajor() {return major;}public void setMajor(String major) {this.major = major;}public String getGrade() {return grade;}public void setGrade(String grade) {this.grade = grade;}public Teacher getSupervisor() {return supervisor;}public void setSupervisor(Teacher supervisor) {this.supervisor = supervisor;}
}
public class Teacher {private int id;    private String name;               // 教师姓名private String gender;        // 教师性别private String researchArea;    // 研究领域private List<Student> supStudents; // 指导学生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 String getGender() {return gender;}public void setGender(String gender) {this.gender = gender;}public String getResearchArea() {return researchArea;}public void setResearchArea(String researchArea) {this.researchArea = researchArea;}public List<Student> getSupStudents() {return supStudents;}public void setSupStudents(List<Student> supStudents) {this.supStudents = supStudents;}
}

3、配置StudentMapper.xml

<?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">
<mapper namespace="org.zsm.myBatis.day02.inner.IStudentOperation"><!-- 定义java Bean的属性与数据库的列之间的映射 --><resultMap type="Student" id="studentResultMap"><id column="id" property="id" /><result column="name" property="name" /><result column="gender" property="gender" /><result column="major" property="major" /><result column="grade" property="grade"/><!-- association处理has-a关系 --><!--注意：这种方式javaType必须指定，表示supervisor的类型是Teacher，否则会报错 --><association property="supervisor" javaType="Teacher"><!-- 教师自身的属性与数据库字段的映射 --><id property="id" column="t_id"/><result property="name" column="t_name"/><result property="gender" column="t_gender"/><result property="researchArea" column="research_area"/></association></resultMap><!-- SQL语句中以"#{}"的形式引用参数 --><select id="getById" parameterType="int" resultMap="studentResultMap">SELECT st.id,st.name,st.gender,st.major,st.grade,t.id t_id,t.name t_name,t.gender t_gender,t.research_area FROM student st, teacher tWHERE st.supervisor_id = t.idAND st.id=#{id}</select>
</mapper>

4、测试

public void testAssociation(){SqlSession session = sqlSessionFactory.openSession();IStudentOperation studentOperation = session.getMapper(IStudentOperation.class);Student student = studentOperation.getById(1);System.out.println(student.getName()+"的指导老师是:"+student.getSupervisor().getName());}

myBatis association的两种形式

一、嵌套的resultMap

    这种方法的本质就是把教师实体映射从association元素中提取出来，用一个resultMap元素表示。然后association元素再引用这个resultMap元素。

<?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">
<mapper namespace="org.zsm.myBatis.day02.inner.IStudentOperation"><!-- 定义java Bean的属性与数据库的列之间的映射 --><resultMap type="Teacher" id="teacherResultMap"><id property="id" column="t_id"/><result property="name" column="t_name"/><result property="gender" column="t_gender"/><result property="researchArea" column="research_area"/></resultMap><resultMap type="Student" id="studentResultMap"><id column="id" property="id" /><result column="name" property="name" /><result column="gender" property="gender" /><result column="major" property="major" /><result column="grade" property="grade"/><!-- 引用teacherResultMap --><association property="supervisor" resultMap="teacherResultMap"/></resultMap><!-- SQL语句中以"#{}"的形式引用参数 --><select id="getById" parameterType="int" resultMap="studentResultMap">SELECT st.id,st.name,st.gender,st.major,st.grade,t.id t_id,t.name t_name,t.gender t_gender,t.research_area FROM student st, teacher tWHERE st.supervisor_id = t.idAND st.id=#{id}</select>
</mapper>

二、嵌套的select语句

    这种方式是使用一条单独的select语句来加载关联的实体(本例中就是教师实体)，然后在association元素中引用此select语句(注：此方法会产生N+1问题，尽量不要用嵌套的select语句)

<?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">
<mapper namespace="org.zsm.myBatis.day02.inner.IStudentOperation"><!-- 定义java Bean的属性与数据库的列之间的映射 --><resultMap type="Teacher" id="supervisorResultMap"><id property="id" column="t_id"/><result property="name" column="t_name"/><result property="gender" column="t_gender"/><result property="researchArea" column="research_area"/></resultMap><resultMap type="Student" id="studentResultMap"><id column="id" property="id" /><result column="name" property="name" /><result column="gender" property="gender" /><result column="major" property="major" /><result column="grade" property="grade"/><!-- 引用teacherResultMap --><association property="supervisor" column="supervisor_id" select="selectSupervisor"/></resultMap><!-- SQL语句中以"#{}"的形式引用参数 --><select id="getById" parameterType="int" resultMap="studentResultMap">select id,name,gender,major,grade,supervisor_id from student where id =#{id}</select><select id="selectSupervisor" parameterType="int" resultMap="supervisorResultMap">select id,name,gender,research_areafrom teacher where id = #{id}</select>
</mapper>