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关于mybatis:Mybatis15-Mybatis一对一多表关联查询

注:代码已托管在 GitHub 上,地址是:https://github.com/Damaer/Mybatis-Learning,我的项目是 mybatis-11-one2one,须要自取,须要配置maven 环境以及 mysql 环境 (sql 语句在 resource 下的 test.sql 中),感觉有用能够点个小星星。

docsify文档地址在:https://damaer.github.io/Mybatis-Learning/#/

所谓一对一多表查问,举个例子:咱们有很多国家,每一个国家只有一个领导人(假如),咱们须要依据 id 查问国家信息,带上领导人的信息。

创立数据表

设计表的时候,咱们须要思考因为是一对多关系,咱们须要在国家表外面应用一个字段对应领导人的信息。

# 创立数据库
CREATE DATABASE `test` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;
#创立数据表
CREATE TABLE `test`.`country` (`cid` INT(10) NOT NULL AUTO_INCREMENT ,`cname` VARCHAR(20) NOT NULL ,`mid` INT(10) NOT NULL,PRIMARY KEY(`cid`)) ENGINE = MyISAM;
CREATE TABLE `test`.`minister` (`mid` INT(10) NOT NULL AUTO_INCREMENT ,`mname` VARCHAR(20) NOT NULL,PRIMARY KEY(`mid`)) ENGINE = MyISAM;

#初始化数据表
INSERT INTO `country` (`cid`, `cname`, `mid`) VALUES ('1', 'aaa', '1'); 
INSERT INTO `country` (`cid`, `cname`, `mid`) VALUES ('2', 'bbb', '2');
INSERT INTO `country` (`cid`, `cname`, `mid`) VALUES ('3', 'ccc', '3'); 

INSERT INTO `minister` (`mid`, `mname`) VALUES ('1', 'sam'); 
INSERT INTO `minister` (`mid`, `mname`) VALUES ('2', 'jane'); 
INSERT INTO `minister` (`mid`, `mname`) VALUES ('3', 'jone'); 

数据表如下:


实体类

country对应的实体类:

public class Country {
    private Integer cid;
    private String cname;
    private Minister minister;

    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;}

    public Minister getMinister() {return minister;}

    public void setMinister(Minister minister) {this.minister = minister;}

    @Override
    public String toString() {
        return "Country{" +
                        "cid=" + cid +
                        ", cname='" + cname + '\'' +
                        ", minister=" + minister +
                        '}';
    }
}

Minister对应的实体类:

public class Minister {
    private Integer mid;
    private String mname;
    @Override
    public String toString() {return "Minister [mid=" + mid + ", mname=" + mname + "]";
    }
    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;}
    
}

MybatisUtils工具类,用于获取单例的 sqlSession 对象:

public class MyBatisUtils {
    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;
    }
}

mybatis.xml 外面注册 mapper 文件:

<mappers>
    <mapper resource="mapper/mapper.xml"/>
</mappers>

咱们的两个接口(对应 mapper 文件中的两个 sql 语句):

public interface ICountryDao {Country selectCountryById(int cid);
    Country selectCountryById2(int cid);
}

mapper文件, 有两种写法,一种是 嵌套后果 ,一种是 嵌套查问

  • 嵌套后果的就是应用一条 sql 实现查问,须要在本人定义的 resultMap 外面应用 <association></association> 来组织后果属性。
  • 嵌套查问也有一个 <association></association>,然而在最里面的<select> 标签并没有将所有须要的货色查问进去,而是在 <association></association> 外面指定须要关联查问的 sql 语句的 id,这样的形式就是 懒加载:
<?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.ICountryDao">
<!--     resultMap 能解决字段和属性不一样的问题 -->
    <!-- 当前用得比拟多,是因为能够应用提早加载 -->
    <!-- 嵌套查问 -->
    <select id="selectMinisterByCountry" resultType="Minister">
    select mid,mname from minister where mid=#{ooo}
    </select>
    <resultMap type="Country" id="countryMapper">
        <id column="cid" property="cid"/>
        <result column="cname" property="cname"/>
        <!-- country 中有一个成员变量是 ministers,它的泛型是 Minister -->
        <association property="minister"
                    select="selectMinisterByCountry"
                    column="mid">
        </association>
    </resultMap>
    <select id="selectCountryById" resultMap="countryMapper">
        select cid,cname,mid
        from country 
        where 
        cid=#{cid}
    </select>

    <!-- 嵌套后果 -->
    <select id="selectCountryById2" resultMap="countryMapper2">
        select * from country c,minister m where c.mid = m.mid and c.cid= #{cid}
    </select>
    <resultMap id="countryMapper2" type="Country">
        <id column="cid" property="cid"/>
        <result column="cname" property="cname"/>
        <association property="minister" javaType="Minister">
            <id property="mid" column="mid"/>
            <result property="mname" column="mname"/>
        </association>
    </resultMap>
</mapper>

单元测试:

public class MyTest {
  private ICountryDao dao;
  private SqlSession sqlSession;
  @Before
  public void Before(){sqlSession=MyBatisUtils.getSqlSession();
    dao=sqlSession.getMapper(ICountryDao.class);
  }
  @Test
  public void TestselectCountryById(){Country country=dao.selectCountryById(1);
    System.out.println(country);
  }

  @Test
  public void TestselectCountryById2(){Country country=dao.selectCountryById2(1);
    System.out.println(country);
  }


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

}

后果:

[service] 2018-07-12 15:23:38,971 - dao.ICountryDao.selectCountryById -537  [main] DEBUG dao.ICountryDao.selectCountryById  - ==>  Preparing: select cid,cname,mid from country where cid=? 
[service] 2018-07-12 15:23:39,004 - dao.ICountryDao.selectCountryById -570  [main] DEBUG dao.ICountryDao.selectCountryById  - ==> Parameters: 1(Integer)
[service] 2018-07-12 15:23:39,037 - dao.ICountryDao.selectMinisterByCountry -603  [main] DEBUG dao.ICountryDao.selectMinisterByCountry  - ====>  Preparing: select mid,mname from minister where mid=? 
[service] 2018-07-12 15:23:39,037 - dao.ICountryDao.selectMinisterByCountry -603  [main] DEBUG dao.ICountryDao.selectMinisterByCountry  - ====> Parameters: 1(Integer)
[service] 2018-07-12 15:23:39,040 - dao.ICountryDao.selectMinisterByCountry -606  [main] DEBUG dao.ICountryDao.selectMinisterByCountry  - <====      Total: 1
[service] 2018-07-12 15:23:39,041 - dao.ICountryDao.selectCountryById -607  [main] DEBUG dao.ICountryDao.selectCountryById  - <==      Total: 1
Country{cid=1, cname='aaa', minister=Minister [mid=1, mname=sam]}

【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,纵使迟缓,驰而不息。这个世界心愿所有都很快,更快,然而我心愿本人能走好每一步,写好每一篇文章,期待和你们一起交换。

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