这里从学生的角度来说就是多对一的场景
那么在Java中是怎么样的呢？
CREATE TABLE `student`( id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, `name` VARCHAR(20) DEFAULT NULL )ENGINE=INNODB DEFAULT CHARSET=utf8; INSERT INTO student VALUES (1,"李老师"); CREATE TABLE `student` ( `id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY, `name` VARCHAR(20) DEFAULT NULL , `tid` INT(4) NOT NULL, KEY `fk_tid` (`tid`), CONSTRAINT fk_tid FOREIGN KEY(`tid`) REFERENCES `teacher`(`id`) INSERT INTO student VALUES (1,"小红",1), (2,"小绿",1), (3,"小灯",1), (4,"小勿",1), (5,"小闯",1);
Student
package pojo;
import lombok.Data;
@Data
public class Student {
private int id;
private String name;
private Teacher teacher;//这里因为我们有一个对应的关系，学生要对应到一个老师，所以要使用Teacher类型
}Teacher
package pojo;
import lombok.Data;
@Data
public class Teacher {
private int id;
private String name;
}（因为这里是多对一，所以我就没有用到TeacherMapper）
StudentMapper
package dao;
import pojo.Student;
import java.util.List;
public interface StudentMapper {
List<Student> getStudents();
List<Student> getStudents2();
}TeacherMapper
package dao;
public interface TeacherMapper {
}
TeacherMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="dao.TeacherMapper">
</mapper>
StudentMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="dao.StudentMapper">
<!-- 第二种 -->
<select id="getStudents2" resultMap="StudentTeacher2">
select s.id sid,s.name sname,t.id tid,t.name tname from student s,teacher t where s.tid=t.id;
</select>
<resultMap id="StudentTeacher2" type="Student" >
<result column="sid" property="id" />
<result column="sname" property="name" />
<association property="teacher" javaType="Teacher">
<result property="id" column="tid" />
<result property="name" column="tname" />
</association>
</resultMap>
<!-- 第一种 -->
<select id="getStudents" resultMap="StudentTeacher">
select * from student;
</select>
<resultMap id="StudentTeacher" type="Student" >
<result column="id" property="id" />
<result column="name" property="name" />
<association property="teacher" column="tid" javaType="Teacher" select="getTeachers" />
</resultMap>
<select id="getTeachers" resultType="Teacher">
select * from teacher where id=#{id};
</select>
</mapper>
这里说明一下
然后这里有两种方法，我们一种一种看
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="dao.StudentMapper">
<!-- 第一种 -->
<select id="getStudents" resultMap="StudentTeacher">
select * from student;
</select>
<resultMap id="StudentTeacher" type="Student" >
<result column="id" property="id" />
<result column="name" property="name" />
<association property="teacher" column="tid" javaType="Teacher" select="getTeachers" />
</resultMap>
<select id="getTeachers" resultType="Teacher">
select * from teacher where id=#{id};
</mapper>也就是这种方法，它其实是相当于我们Sql里面的子查询，也就是一层查询里面嵌套一层查询
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="dao.StudentMapper">
<!-- 第二种 -->
<select id="getStudents2" resultMap="StudentTeacher2">
select s.id sid,s.name sname,t.id tid,t.name tname from student s,teacher t where s.tid=t.id;
</select>
<resultMap id="StudentTeacher2" type="Student" >
<result column="sid" property="id" />
<result column="sname" property="name" />
<association property="teacher" javaType="Teacher">
<result property="id" column="tid" />
<result property="name" column="tname" />
</association>
</resultMap>
</mapper>
这种方法就是我们一次性把结果查询出来然后对结果集做一下处理
这里特别说明一下column这个属性，这个可不是数据库中你需要的字段名，而是你查询出来的那张表的字段名，这里起了别名之后就不再是原来teacher数据库表中的字段名了
