多对一的理解：

• 多个学生对应一个老师
• 如果对于学生这边，就是一个多对一的现象，即从学生这边关联一个老师！

1、数据库设计

CREATE TABLE `teacher` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8
INSERT INTO teacher(`id`, `name`) VALUES (1, '秦老师'); 
CREATE TABLE `student` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  `tid` INT(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fktid` (`tid`),
  CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('1', '小明', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('2', '小红', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('3', '小张', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('4', '小李', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('5', '小王', '1');

2、搭建测试环境

2.1、编写实体类

Student

package com.study.pojo;

public class Student {
    private int id;
    private String name;
    private Teacher teacher;
}

Teacher

package com.study.pojo;

public class Teacher {
    private int id;
    private String name;
}

2.2、编写实体类对应的Mapper接口

// Student
public interface StudentMapper {
}
// Teache
public interface TeacherMapper {
}

2.3、编写Mapper接口对应的 mapper.xml配置文件

Student

<?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="com.study.mapper.StudentMapper">
 
</mapper>

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">
<mapper namespace="com.study.mapper.TeacherMapper">

</mapper>

3、按结果嵌套处理

1. 接口方法编写

   public List<Student> getStudents2();
   

2. 编写对应的mapper文件

   <!--
   按查询结果嵌套处理
   思路：
       1. 直接查询出结果，进行结果集的映射
   -->
   <select id="getStudents2" resultMap="StudentTeacher2" >
       select s.id sid, s.name sname , t.name tname
       from student s,teacher t
       where s.tid = t.id
   </select>
   <resultMap id="StudentTeacher2" type="Student">
       <id property="id" column="sid"/>
       <result property="name" column="sname"/>
       <!--关联对象property 关联对象在Student实体类中的属性-->
       <association property="teacher" javaType="Teacher">
           <result property="name" column="tname"/>
       </association>
   </resultMap>
   

3. 测试

   @Test
   public void getStudentList2(){
       SqlSession sqlSession = mybatisUtils.getSqlSession();
       StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
       List<Student> studentList = studentMapper.getStudentList2();
       for (Student student : studentList){
           System.out.println(student);
       }
       sqlSession.close();
   }
   

4、按查询嵌套处理

1. 接口方法编写

   public List<Student> getStudents();
   

2. 编写对应的mapper文件

   <resultMap id="StudentTeacher" type="Student">
       <!--association关联属性  property属性名 javaType属性类型 column在多的一方的表中的列名-->
       <association property="teacher"  column="{id=tid,name=tid}" javaType="Teacher" select="getTeacher"/>
   </resultMap>
   <!--
   这里传递过来的id，只有一个属性的时候，下面可以写任何值
   association中column多参数配置：
       column="{key=value,key=value}"
       其实就是键值对的形式，key是传给下个sql的取值名称，value是片段一中sql查询的字段名。
   -->
   <select id="getTeacher" resultType="teacher">
       select * from teacher where id = #{id} and name = #{name}
   </select>
   

3. 测试

   @Test
   public void getStudentList(){
       SqlSession sqlSession = mybatisUtils.getSqlSession();
       StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
       List<Student> studentList = studentMapper.getStudentList();
       for (Student student : studentList){
           System.out.println(student);
       }
       sqlSession.close();
   }
   

5、小结

• 按照查询进行嵌套处理就像SQL中的子查询
• 按照结果进行嵌套处理就像SQL中的联表查询

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