Mybatis自关联查询一对多查询的实现示例

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 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：

     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--> select id,name from newslabel where pid=#{ooo}   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.INewsLabelDa来源gaodai#ma#com搞*!代#%^码$网o.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=[]]]]]]

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