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Mybatis【16】-- Mybatis 多对一关联查询

发布于: 2021 年 01 月 30 日

注:代码已托管在GitHub上,地址是:https://github.com/Damaer/Mybatis-Learning ,项目是mybatis-12-many2one,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/#/


一对多关联查询:每个国家有很多大臣,领导人,现在我们查询一个领导,希望能将他所在国家的信息连带出来,这就是一对多关联查询。


数据表如下:

国家表:


领导人表:


创建数据库/表

#创建数据库CREATE DATABASE `test` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;#创建数据表CREATE TABLE `test`.`country` ( `cid` INT(10) NOT NULL AUTO_INCREMENT ,`cname` VARCHAR(20) NOT NULL ,PRIMARY KEY(`cid`)) ENGINE = MyISAM;CREATE TABLE `test`.`minister` ( `mid` INT(10) NOT NULL AUTO_INCREMENT ,`mname` VARCHAR(20) NOT NULL ,`countryId` INT(20) NOT NULL ,PRIMARY KEY(`mid`)) ENGINE = MyISAM;
#初始化数据表INSERT INTO `country` (`cid`, `cname`) VALUES ('1', 'USA');INSERT INTO `country` (`cid`, `cname`) VALUES ('2', 'England');
INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('1', 'aaa', '1');INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('2', 'bbb', '1');INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('3', 'ccc', '2');INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('4', 'ddd', '2');INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('5', 'eee', '2');
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实体类

country类:

public class Country {	private Integer cid;	private String cname;
public Integer getCid() { return cid; }
public void setCid(Integer cid) { this.cid = cid; }
public String getCname() { return cname; }
public void setCname(String cname) { this.cname = cname; }
@Override public String toString() { return "Country{" + "cid=" + cid + ", cname='" + cname + '\'' + '}'; }}
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Minister类,在这个类里面使用Country当属性。

public class Minister {	private Integer mid;	private String mname;	private Country country;
public Integer getMid() { return mid; }
public void setMid(Integer mid) { this.mid = mid; }
public String getMname() { return mname; }
public void setMname(String mname) { this.mname = mname; }
public Country getCountry() { return country; }
public void setCountry(Country country) { this.country = country; }
@Override public String toString() { return "Minister{" + "mid=" + mid + ", mname='" + mname + '\'' + ", country=" + country + '}'; }}
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Mybatis工具类,获取sqlsession实例:

public class MyBatisUtils {
/* public SqlSession getSqlSession(){ InputStream is; try { is = Resources.getResourceAsStream("mybatis.xml"); return new SqlSessionFactoryBuilder().build(is).openSession(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } return null; }*/ static private SqlSessionFactory sqlSessionFactory;
static public SqlSession getSqlSession() { InputStream is; try { is = Resources.getResourceAsStream("mybatis.xml"); if (sqlSessionFactory == null) { sqlSessionFactory = new SqlSessionFactoryBuilder().build(is); } return sqlSessionFactory.openSession(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } return null; }}
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mybatis.xml主配置文件里面注册mapper文件:

<mappers>    <mapper resource="mapper/mapper.xml"/></mappers>
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接口的声明:

public interface IMinisterDao {	Minister selectMinisterById(int mid);	Minister selectMinisterById2(int mid);}
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mapper文件最重要的部分,与一对一有点像,两种查询方式,一种是嵌套结果,一种是嵌套查询,嵌套查询可以使用懒加载模式,比较常用:

<?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.IMinisterDao"><!-- 	resultMap 能解决字段和属性不一样的问题 -->	<!-- 以后用得比较多 ,是因为可以使用延迟加载-->	<!-- 嵌套查询 -->	<select id="selectCountryById" resultType="Country">	select * from country where cid=#{countryId}	</select>	<resultMap type="Minister" id="countryMapper">		<id column="mid" property="mid"/>		<result column="mname" property="mname"/>		<!-- country中有一个成员变量是ministers,它的泛型是Minister -->		<collection property="country"					ofType="Country"					select="selectCountryById"					column="countryId">		</collection>	</resultMap>	<select id="selectMinisterById" resultMap="countryMapper">		select mid,mname,countryId		from minister		where 		mid=#{mid}	</select>
<!-- 嵌套结果--> <select id="selectMinisterById2" resultMap="countryMapper2"> select mid,mname,countryId,cid,cname from minister,country where mid=#{mid} and countryId = cid </select> <resultMap id="countryMapper2" type="Minister"> <id column="mid" property="mid"/> <result column="mname" property="mname"/> <collection property="country" ofType="Country"> <id property="cid" column="cid"/> <result property="cname" column="cname"/> </collection> </resultMap></mapper>
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单元测试:

public class MyTest {  private IMinisterDao dao;  private SqlSession sqlSession;  @Before  public void Before(){    sqlSession=MyBatisUtils.getSqlSession();    dao=sqlSession.getMapper(IMinisterDao.class);  }  @Test  public void TestselectMinisterById(){    Minister minister = dao.selectMinisterById(1);    System.out.println(minister);  }
@Test public void TestselectMinisterById2(){ Minister minister = dao.selectMinisterById2(1); System.out.println(minister); }

@After public void after(){ if(sqlSession!=null){ sqlSession.close(); } }
}
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结果:

[service] 2018-07-12 20:07:23,875 - dao.IMinisterDao.selectMinisterById2 -567  [main] DEBUG dao.IMinisterDao.selectMinisterById2  - ==>  Preparing: select mid,mname,countryId,cid,cname from minister,country where mid=? and countryId = cid [service] 2018-07-12 20:07:23,909 - dao.IMinisterDao.selectMinisterById2 -601  [main] DEBUG dao.IMinisterDao.selectMinisterById2  - ==> Parameters: 1(Integer)[service] 2018-07-12 20:07:23,930 - dao.IMinisterDao.selectMinisterById2 -622  [main] DEBUG dao.IMinisterDao.selectMinisterById2  - <==      Total: 1Minister{mid=1, mname='aaa', country=Country{cid=1, cname='USA'}}
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【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,纵使缓慢,驰而不息。这个世界希望一切都很快,更快,但是我希望自己能走好每一步,写好每一篇文章,期待和你们一起交流。


发布于: 2021 年 01 月 30 日阅读数: 15
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纵使缓慢,驰而不息。 2018.05.17 加入

慢慢走,比较快。公众号:秦怀杂货店

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Mybatis【16】-- Mybatis多对一关联查询