多对一的理解：
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, '吴老师');
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');【Lombok的使用】
1. IDEA安装Lombok插件
2. 引入Maven依赖
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.16.10</version>
</dependency>3. 在代码中增加注解
@Data
public class Teacher {
private int id;
private String name;
}@Data
public class Student {
private int id;
private String name;
//多个学生可以是同一个老师，即多对一
private Teacher teacher;
}4. 编写实体类对应的Mapper接口 【两个】
无论有没有需求，都应该写上，以备后来之需！
public interface StudentMapper {
}public interface TeacherMapper {
}5. 编写Mapper接口对应的 mapper.xml配置文件 【两个】
无论有没有需求，都应该写上，以备后来之需！
<?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.hang.mapper.StudentMapper">
</mapper><?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.hang.mapper.TeacherMapper">
</mapper>1. 给StudentMapper接口增加方法
//获取所有学生及对应老师的信息 public List<Student> getStudents();
2. 编写对应的Mapper文件
<?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.hang.mapper.StudentMapper">
<select id="getStudents" resultMap="studentTeacher">
select * from mybatis.student;
</select>
<resultMap id="studentTeacher" type="student">
<result property="id" column="id"/>
<result property="name" column="name"/>
<!--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="tid" javaType="teacher" select="getTeacher"/>
</resultMap>
<select id="getTeacher" resultType="teacher">
select * from mybatis.teacher;
</select>
</mapper>3. 编写完毕去Mybatis配置文件中，注册Mapper！
4.测试
@Test
public void getStudents(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
List<Student> students= mapper.getStudents();
for (Student student : students) {
System.out.println(
"学生名:"+ student.getName()
+"\t老师:"+student.getTeacher().getName());
}
sqlSession.close();
}除了上面这种方式，还有其他思路吗？
我们还可以按照结果进行嵌套处理；
1. 接口方法编写
public List<Student> getStudents2();
2. 编写对应的mapper文件
<select id="getStudents2" resultMap="studentTeacher2">
select s.id sid, s.name sname, t.id tid, t.name tname
from mybatis.student s, mybatis.teacher t
where t.id = s.tid
</select><br>
<resultMap id="studentTeacher2" type="student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<association property="teacher" javaType="teacher">
<result property="id" column="tid"/>
<result property="name" column="tname"/>
</association>
</resultMap>3. 去mybatis-config文件中注入【此处应该处理过了】
4. 测试
@Test
public void getStudents2(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
List<Student> students= mapper.getStudents2();
for (Student student : students) {
System.out.println(
"学生名:"+ student.getName()
+"\t老师:"+student.getTeacher().getName());
}
sqlSession.close();
} 5、小结
一对多的理解：
一个老师拥有多个学生如果对于老师这边，就是一个一对多的现象，即从一个老师下面拥有一群学生（集合）！
@Data
public class Student {
private int id;
private String name;
private int tid;
}@Data
public class Teacher {
private int id;
private String name;
//一个老师多个学生
private List<Student> students;
}和之前一样，搭建测试的环境！
1. TeacherMapper接口编写方法
//获取指定老师，及老师下的所有学生 public Teacher getTeacher(int id);
2. 编写接口对应的Mapper配置文件
<select id="getTeacher" resultMap="TeacherStudent">
select s.id sid, s.name sname, t.id tid, t.name tname
from mybatis.teacher t, mybatis.student s
where t.id = s.tid and t.id = #{tid};
</select>
<resultMap id="TeacherStudent" type="teacher">
<result property="id" column="tid"/>
<result property="name" column="tname"/>
<collection property="students" ofType="student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<result property="tid" column="tid"/>
</collection>
</resultMap>3. 将Mapper文件注册到MyBatis-config文件中
4. 测试
@Test
public void testGetTeacher(){
SqlSession session = MybatisUtils.getSession();
TeacherMapper mapper = session.getMapper(TeacherMapper.class);
Teacher teacher = mapper.getTeacher(1);
System.out.println(teacher.getName());
System.out.println(teacher.getStudents());
}1. TeacherMapper接口编写方法
public Teacher getTeacher2(int id);
2. 编写接口对应的Mapper配置文件
<select id="getTeacher2" resultMap="TeacherStudent2">
select * from teacher where id = #{id}
</select>
<resultMap id="TeacherStudent2" type="Teacher">
<!--column是一对多的外键 , 写的是一的主键的列名-->
<collection property="students" javaType="ArrayList" ofType="Student" column="id" select="getStudentByTeacherId"/>
</resultMap>
<select id="getStudentByTeacherId" resultType="Student">
select * from student where tid = #{id}
</select>3. 将Mapper文件注册到MyBatis-config文件中
4. 测试
@Test
public void testGetTeacher2(){
SqlSession session = MybatisUtils.getSession();
TeacherMapper mapper = session.getMapper(TeacherMapper.class);
Teacher teacher = mapper.getTeacher2(1);
System.out.println(teacher.getName());
System.out.println(teacher.getStudents());
}关联-association集合-collection所以association是用于一对一和多对一，而collection是用于一对多的关系JavaType和ofType都是用来指定对象类型的
注意说明：
