（十七）Mybatis自关联查询一对多查询

注：代码已托管在GitHub上，地址是：https://github.com/Damaer/Mybatis-Learning ，项目是mybatis-13-oneself-one2many，需要自取，需要配置maven环境以及mysql环境(sql语句在resource下的test.sql中)，觉得有用可以点个小星星，小菜鸟在此Thanks~
所谓自关联查询，是指自己既然充当一方，又充当多方。比如新闻栏目的数据表，自己可以是父栏目，也可以是多方，子栏目。在数据表里面实现就是一张表，有一个外键pid，用来表示该栏目的父栏目，一级栏目没有父栏目的，可以将其外键设置为0.
DB表如下：

查询指定栏目的所有子孙栏目

查询指定目录的所有子孙目录，我们需要使用递归的思想，查出当前栏目之后，需要将当前栏目的id作为下一级栏目的pid。
实体类NewsLabel.java,使用一对多的关系：

import java.util.Set;

public class NewsLabel {
    private Integer id;
    private String name;
    private Setchildren;
    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 getChildren() {
        return children;
    }
    public void setChildren(Set 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 selectChildByParentId(int pid);
}

mapper.xml文件，在递归里面使用本身sql：

"1.0" encoding="UTF-8"?>
"-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
"dao.INewsLabelDao">
    "beans.NewsLabel" id="newsLabelMapper">
        "id" property="id"/>
        "name" property="name"/>
        "children" 
                ofType="NewsLabel"
                select="selectChildByParentId"
                column="id"/>
    
    "selectChildByParentId" resultMap="newsLabelMapper">
        select id,name from newslabel where pid=#{xxx}
    

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(){
    Listchildren=dao.selectChildByParentId(2);
    for(NewsLabel newsLabel:children){
      System.out.println(newsLabel);
    }
  }
  @After
  public void after(){
    if(sqlSession!=null){
      sqlSession.close();
    }
  }

}

结果：

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，不能将自己的信息输出。

查询指定目录以及指定子孙目录

添加一个sql的接口：

  List selectSelfAndChildByParentId(int pid);

mapper文件里面实现,在resultMap里面递归调用另一个sql，最外层的sql只执行一次，这样就可以实现查询自身一次，递归查询子孙栏目的功能：

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

单元测试：

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

结果：

[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: 1
NewsLabel [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=[]]]]]]