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。
docsify文档地址在：https://damaer.github.io/Mybatis-Learning/#/
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 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，在递归里面使用本身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(); } }}
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<NewsLabel> 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--> <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); } }
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: 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=[]]]]]]
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