注：代码已托管在GitHub上，地址是：https://github.com/Damaer/Mybatis-Learning ，我的项目是mybatis-13-oneself-one2many，须要自取，须要配置maven环境以及mysql环境(sql语句在resource下的test.sql中)，感觉有用能够点个小星星。

docsify文档地址在：https://damaer.github.io/Mybatis-Learning/#/

所谓自关联查问，是指本人既然充当一方，又充当多方。比方新闻栏目的数据表，本人能够是父栏目，也能够是多方，子栏目。在数据表外面实现就是一张表，有一个外键pid，用来示意该栏目的父栏目，一级栏目没有父栏目的，能够将其外键设置为0。

DB表如下：

查问指定栏目的所有子孙栏目

查问指定目录的所有子孙目录，咱们须要应用递归的思维，查出以后栏目之后，须要将以后栏目的id作为下一级栏目的pid。

实体类NewsLabel.java,应用一对多的关系：

import java.util.Set;public class NewsLabel {    private Integer id;    private String name;    private Set<NewsLabel>children;    public Integer getId() {        return id;    }    public void setId(Integer id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public Set<NewsLabel> getChildren() {        return children;    }    public void setChildren(Set<NewsLabel> children) {        this.children = children;    }    @Override    public String toString() {        return "NewsLabel [id=" + id + ", name=" + name + ", children="                + children + "]";    }    }

定义sql接口:

public interface INewsLabelDao {    List<NewsLabel> selectChildByParentId(int pid);}

mapper.xml文件，在递归外面应用自身sql：

<?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="dao.INewsLabelDao">    <resultMap type="beans.NewsLabel" id="newsLabelMapper">        <id column="id" property="id"/>        <result column="name" property="name"/>        <collection property="children"                 ofType="NewsLabel"                select="selectChildByParentId"                column="id"/>    </resultMap>    <select id="selectChildByParentId" resultMap="newsLabelMapper">        select id,name from newslabel where pid=#{xxx}    </select></mapper>

测试类MyTest.java:

public class MyTest {  private INewsLabelDao dao;  private SqlSession sqlSession;  @Before  public void Before(){    sqlSession=MyBatisUtils.getSqlSession();    dao=sqlSession.getMapper(INewsLabelDao.class);  }  @Test  public void TestselectMinisterById(){    List<NewsLabel>children=dao.selectChildByParentId(2);    for(NewsLabel newsLabel:children){      System.out.println(newsLabel);    }  }  @After  public void after(){    if(sqlSession!=null){      sqlSession.close();    }  }}

后果：

NewsLabel [id=3, name=NBA, children=[NewsLabel [id=5, name=火箭, children=[]], NewsLabel [id=6, name=湖人, children=[]]]]NewsLabel [id=4, name=CBA, children=[NewsLabel [id=7, name=北京金瓯, children=[]], NewsLabel [id=8, name=浙江广夏, children=[]], NewsLabel [id=9, name=青岛双星, children=[]]]]

这样的写法只能选出子孙栏目，不能将本人的信息输入。

查问指定目录以及指定子孙目录

增加一个sql的接口：

List<NewsLabel> selectSelfAndChildByParentId(int pid);

mapper文件外面实现,在resultMap外面递归调用另一个sql，最外层的sql只执行一次，这样就能够实现查问本身一次，递归查问子孙栏目的性能：

    <!--  筛选出本人以及子孙栏目-->    <select id="selectChildByParentId2" resultMap="newsLabelMapper2">        select id,name from newslabel where pid=#{ooo}    </select>    <resultMap type="beans.NewsLabel" id="newsLabelMapper2">        <id column="id" property="id"/>        <result column="name" property="name"/>        <collection property="children"                    ofType="NewsLabel"                    select="selectChildByParentId2"                    column="id"/>    </resultMap>    <select id="selectSelfAndChildByParentId" resultMap="newsLabelMapper2">        select id,name from newslabel where id=#{xxx}    </select>

单元测试：

  @Test  public void TestselectSelfAndChildrenLabelById(){    List<NewsLabel> children = dao.selectSelfAndChildByParentId(2);    for (NewsLabel newsLabel : children) {      System.out.println(newsLabel);    }  }

后果：

[service] 2018-07-16 11:17:16,667 - org.apache.ibatis.transaction.jdbc.JdbcTransaction -450  [main] DEBUG org.apache.ibatis.transaction.jdbc.JdbcTransaction  - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@5bb21b69][service] 2018-07-16 11:17:16,669 - dao.INewsLabelDao.selectSelfAndChildByParentId -452  [main] DEBUG dao.INewsLabelDao.selectSelfAndChildByParentId  - ==>  Preparing: select id,name from newslabel where id=? [service] 2018-07-16 11:17:16,704 - dao.INewsLabelDao.selectSelfAndChildByParentId -487  [main] DEBUG dao.INewsLabelDao.selectSelfAndChildByParentId  - ==> Parameters: 2(Integer)[service] 2018-07-16 11:17:16,722 - dao.INewsLabelDao.selectChildByParentId2 -505  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ====>  Preparing: select id,name from newslabel where pid=? [service] 2018-07-16 11:17:16,723 - dao.INewsLabelDao.selectChildByParentId2 -506  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ====> Parameters: 2(Integer)[service] 2018-07-16 11:17:16,726 - dao.INewsLabelDao.selectChildByParentId2 -509  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ======>  Preparing: select id,name from newslabel where pid=? [service] 2018-07-16 11:17:16,726 - dao.INewsLabelDao.selectChildByParentId2 -509  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ======> Parameters: 3(Integer)[service] 2018-07-16 11:17:16,727 - dao.INewsLabelDao.selectChildByParentId2 -510  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ========>  Preparing: select id,name from newslabel where pid=? [service] 2018-07-16 11:17:16,728 - dao.INewsLabelDao.selectChildByParentId2 -511  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ========> Parameters: 5(Integer)[service] 2018-07-16 11:17:16,729 - dao.INewsLabelDao.selectChildByParentId2 -512  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - <========      Total: 0[service] 2018-07-16 11:17:16,732 - dao.INewsLabelDao.selectChildByParentId2 -515  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ========>  Preparing: select id,name from newslabel where pid=? [service] 2018-07-16 11:17:16,732 - dao.INewsLabelDao.selectChildByParentId2 -515  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ========> Parameters: 6(Integer)[service] 2018-07-16 11:17:16,733 - dao.INewsLabelDao.selectChildByParentId2 -516  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - <========      Total: 0[service] 2018-07-16 11:17:16,734 - dao.INewsLabelDao.selectChildByParentId2 -517  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - <======      Total: 2[service] 2018-07-16 11:17:16,734 - dao.INewsLabelDao.selectChildByParentId2 -517  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ======>  Preparing: select id,name from newslabel where pid=? [service] 2018-07-16 11:17:16,734 - dao.INewsLabelDao.selectChildByParentId2 -517  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ======> Parameters: 4(Integer)[service] 2018-07-16 11:17:16,736 - dao.INewsLabelDao.selectChildByParentId2 -519  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ========>  Preparing: select id,name from newslabel where pid=? [service] 2018-07-16 11:17:16,736 - dao.INewsLabelDao.selectChildByParentId2 -519  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ========> Parameters: 7(Integer)[service] 2018-07-16 11:17:16,738 - dao.INewsLabelDao.selectChildByParentId2 -521  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - <========      Total: 0[service] 2018-07-16 11:17:16,738 - dao.INewsLabelDao.selectChildByParentId2 -521  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ========>  Preparing: select id,name from newslabel where pid=? [service] 2018-07-16 11:17:16,739 - dao.INewsLabelDao.selectChildByParentId2 -522  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ========> Parameters: 8(Integer)[service] 2018-07-16 11:17:16,741 - dao.INewsLabelDao.selectChildByParentId2 -524  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - <========      Total: 0[service] 2018-07-16 11:17:16,742 - dao.INewsLabelDao.selectChildByParentId2 -525  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ========>  Preparing: select id,name from newslabel where pid=? [service] 2018-07-16 11:17:16,742 - dao.INewsLabelDao.selectChildByParentId2 -525  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - ========> Parameters: 9(Integer)[service] 2018-07-16 11:17:16,743 - dao.INewsLabelDao.selectChildByParentId2 -526  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - <========      Total: 0[service] 2018-07-16 11:17:16,744 - dao.INewsLabelDao.selectChildByParentId2 -527  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - <======      Total: 3[service] 2018-07-16 11:17:16,744 - dao.INewsLabelDao.selectChildByParentId2 -527  [main] DEBUG dao.INewsLabelDao.selectChildByParentId2  - <====      Total: 2[service] 2018-07-16 11:17:16,745 - dao.INewsLabelDao.selectSelfAndChildByParentId -528  [main] DEBUG dao.INewsLabelDao.selectSelfAndChildByParentId  - <==      Total: 1NewsLabel [id=2, name=体育新闻, children=[NewsLabel [id=3, name=NBA, children=[NewsLabel [id=6, name=湖人, children=[]], NewsLabel [id=5, name=火箭, children=[]]]], NewsLabel [id=4, name=CBA, children=[NewsLabel [id=7, name=北京金瓯, children=[]], NewsLabel [id=8, name=浙江广夏, children=[]], NewsLabel [id=9, name=青岛双星, children=[]]]]]]

【作者简介】：
秦怀，公众号【秦怀杂货店】作者，技术之路不在一时，山高水长，纵使迟缓，驰而不息。

此文章仅代表本人（本菜鸟）学习积攒记录，或者学习笔记，如有侵权，请分割作者核实删除。人无完人，文章也一样，文笔稚嫩，在下不才，勿喷，如果有谬误之处，还望指出，感激不尽~