Mybatis【15】-- Mybatis一對一多表關聯查詢

注:代碼已託管在GitHub上,地址是:https://github.com/Damaer/Mybatis-Learning ,項目是mybatis-11-one2one,須要自取,須要配置maven環境以及mysql環境(sql語句在resource下的test.sql中),以爲有用能夠點個小星星。java

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

所謂一對一多表查詢,舉個例子:咱們有不少國家,每個國家只有一個領導人(假設),咱們須要根據id查詢國家信息,帶上領導人的信息。git

建立數據表

設計表的時候,咱們須要考慮因爲是一對多關係,咱們須要在國家表裏面使用一個字段對應領導人的信息。github

#建立數據庫
CREATE DATABASE `test` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;
#建立數據表
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;

#初始化數據表
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');

數據表以下:sql


實體類

country對應的實體類:shell

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對應的實體類:數據庫

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工具類,用於獲取單例的sqlSession對象:mybatis

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裏面註冊mapper文件:app

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

咱們的兩個接口(對應mapper文件中的兩個sql語句):maven

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

mapper文件,有兩種寫法,一種是嵌套結果,一種是嵌套查詢

  • 嵌套結果的就是使用一條sql完成查詢,須要在本身定義的resultMap裏面使用<association></association>來組織結果屬性。

  • 嵌套查詢也有一個<association></association>,可是在最外面的<select>標籤並無將全部須要的東西查詢出來,而是在<association></association>裏面指定須要關聯查詢的sql語句的id,這樣的方式就是懶加載:

<?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 能解決字段和屬性不同的問題 -->
	<!-- 之後用得比較多 ,是由於可使用延遲加載-->
	<!-- 嵌套查詢 -->
	<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中有一個成員變量是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>

    <!-- 嵌套結果-->
    <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>

單元測試:

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();
    }
  }

}

結果:

[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]}

【做者簡介】
秦懷,公衆號【秦懷雜貨店】做者,技術之路不在一時,山高水長,縱使緩慢,馳而不息。這個世界但願一切都很快,更快,可是我但願本身能走好每一步,寫好每一篇文章,期待和大家一塊兒交流。

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