Mybatis 多级分类查询

@Data
public class EduOneSubjectVO {
    @ApiModelProperty(value = "课程类别ID")
    private String id;
    @ApiModelProperty(value = "类别名称")
    private String label;
    @ApiModelProperty(value = "二级分类list内容")
    private  List<EduOneSubjectVO>  children = new ArrayList<>();
}
mapper
<mapper namespace="com.atguigu.eduservice.mapper.EduSubjectMapper">
    <resultMap id="queryAllSubjectMap" type="com.atguigu.eduservice.entity.vo.EduOneSubjectVO">
        <id property="id" column="oneId" />
        <result property="label" column="oneTitle"/>
        <collection property="children" ofType="com.atguigu.eduservice.entity.vo.EduOneSubjectVO">
            <id property="id" column="twoId"/>
            <result property="label" column="twoTitle"/>
        </collection>
    </resultMap>
    <select id="queryAllSubject" resultType="list" resultMap="queryAllSubjectMap">
        SELECT o.id as oneId,o.title as oneTitle,t.id as twoId,t.title as twoTitle
        FROM edu_subject o
                 LEFT JOIN edu_subject t
                           on o.id = t.parent_id  WHERE o.parent_id = 0
    </select>
service
List<EduOneSubjectVO> queryAllSubject();
impl
@Override
public List<EduOneSubjectVO> queryAllSubject() {
    List<EduOneSubjectVO> eduOneSubjectVOS = subjectMapper.queryAllSubject();
    Collections.sort(eduOneSubjectVOS, new Comparator<EduOneSubjectVO>() {
        @Override
        public int compare(EduOneSubjectVO o1, EduOneSubjectVO o2) {
            return o1.getChildren().size() - o2.getChildren().size() >= 0? -1:1;
        }
    });
    return eduOneSubjectVOS;
}
controller
@GetMapping("getAllSubject")
@ApiOperation(value = "获取课程分类")
public R getAllSubject(){
    List<EduOneSubjectVO> list = subjectService.queryAllSubject();
    return R.ok().data("list",list);
}