Mybaits一对多处理。

一对多：

比如：一个老师拥有多个学生！
对于老师而言，就是一对多的关系！

上一节，多对一地址：点击访问

数据库（和上一篇Mybatis多对一的处理一样）

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, '庞老师');

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');

实体类（省略了构造函数、Setter/Getter）

//Teacher类
public class Teacher {
    private int id;
    private String name;

    //一个老师拥有多个学生
    private List<Student> students;
    
}

//Student类
public class Student {
    private int id;
    private String name;
    private int tid;

}

TeacherMapper接口（interface）

//获取指定老师下的所有学生以及老师的信息
    Teacher getTeacher(@Param("tid") int id);

    Teacher getTeacher2(@Param("tid") int id);

按照查询嵌套处理

<select id="getTeacher2" resultMap="TeacherStudent2">
    select * from mybatis.teacher where id = #{tid}
</select>

<resultMap id="TeacherStudent2" type="Teacher">
    <collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id"/>
</resultMap>

<select id="getStudentByTeacherId" resultType="Student">
    select * from mybatis.student where tid = #{tid}
</select>

按照结果嵌套处理

<!--按结果嵌套查询-->
<select id="getTeacher" resultMap="TeacherStudent">
    select s.id sid,s.name sname,t.name tname,t.id tid
    from student s,teacher t
    where s.tid = t.id and t.id = #{tid}
</select>

<resultMap id="TeacherStudent" type="Teacher">
    <result property="id" column="tid"/>
    <result property="name" column="tname"/>
    <!--复杂的属性
    对象：association
    集合：collection
    集合中的泛型信息，使用ofType获取
    -->
    <collection property="students" ofType="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <result property="tid" column="tid"/>
    </collection>
</resultMap>

测试类

@Test
    public void getTeacher(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher(1);

        System.out.println(teacher);

        sqlSession.close();
    }

@Test
    public void getTeacher2(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher2(1);

        System.out.println(teacher);

        sqlSession.close();
    }