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注：代碼已托管在GitHub上，地址是：https://github.com/Damaer/Mybatis-Learning ，項(xiàng)目是mybatis-13-oneself-one2many，需要自取，需要配置maven環(huán)境以及mysql環(huán)境(sql語(yǔ)句在resource下的test.sql中)，覺(jué)得有用可以點(diǎn)個(gè)小星星。

docsify文檔地址在：https://damaer.github.io/Mybatis-Learning/#/

所謂自關(guān)聯(lián)查詢，是指自己既然充當(dāng)一方，又充當(dāng)多方。比如新聞欄目的數(shù)據(jù)表，自己可以是父欄目，也可以是多方，子欄目。在數(shù)據(jù)表里面實(shí)現(xiàn)就是一張表，有一個(gè)外鍵pid，用來(lái)表示該欄目的父欄目，一級(jí)欄目沒(méi)有父欄目的，可以將其外鍵設(shè)置為0。

DB表如下：

Mybatis自關(guān)聯(lián)查詢一對(duì)多查詢的實(shí)現(xiàn)示例

查詢指定欄目的所有子孫欄目

查詢指定目錄的所有子孫目錄，我們需要使用遞歸的思想，查出當(dāng)前欄目之后，需要將當(dāng)前欄目的id作為下一級(jí)欄目的pid。

實(shí)體類NewsLabel.java,使用一對(duì)多的關(guān)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 + "]";
}
}

定義sql接口:

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

mapper.xml文件，在遞歸里面使用本身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>

測(cè)試類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();
}
}
}

結(jié)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=北京金甌, children=[]], NewsLabel [id=8, name=浙江廣夏, children=[]], NewsLabel [id=9, name=青島雙星, children=[]]]]

這樣的寫法只能選出子孫欄目，不能將自己的信息輸出。

添加一個(gè)sql的接口：

List<NewsLabel> selectSelfAndChildByParentId(int pid);

mapper文件里面實(shí)現(xiàn),在resultMap里面遞歸調(diào)用另一個(gè)sql，最外層的sql只執(zhí)行一次，這樣就可以實(shí)現(xiàn)查詢自身一次，遞歸查詢子孫欄目的功能：

<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>

單元測(cè)試：

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

結(jié)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=體育新聞, 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=北京金甌, children=[]], NewsLabel [id=8, name=浙江廣夏, children=[]], NewsLabel [id=9, name=青島雙星, children=[]]]]]]

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原文鏈接：https://segmentfault.com/a/1190000039134263