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'); 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 + '\'' +
'}';
}
} 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 +
'}';
}
} 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;
}
} 在mybatis.xml主配置文件里面注册mapper文件：
<mappers>
<mapper resource="mapper/mapper.xml"/>
</mappers> 接口的声明：
public interface IMinisterDao {
Minister selectMinisterById(int mid);
Minister selectMinisterById2(int mid);
} 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> 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();
}
}
} 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: 1
Minister{mid=1, mname='aaa', country=Country{cid=1, cname='USA'}} 【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。
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