3.1 多对一关联查询 - 通过多表连接查询方式实现

以部长Minister与国家Countr与间的多对一关系而言，查询部长时，将这个部长所属的国家查询出来。

DB数据模型不变，还是在多表minister中定义外键。

定义实体类：Country.java

package com.bjpowernode.beans;

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

    public Country() {
        super();
    }

    ...
    getter and setter methods
    ...        
}

定义实体类：Minister.java

package com.bjpowernode.beans;

public class Minister {
    private Integer mid;
    private String mname;
    //关联属性
    private Country country;

    public Minister() {
        super();
    }

    public Minister(Integer mid, String mname) {
        super();
        this.mid = mid;
        this.mname = mname;
    }

    ...
    getter and setter methods
    ...    
}

接口

package com.bjpowernode.dao;

public interface IMinisterDao {
    Minister selectMinisterById(int mid);
}

映射文件：

<?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="com.bjpowernode.dao.IMinisterDao">

    <resultMap type="Minister" id="ministerMapper">
        <id column="mid" property="mid"/>
        <result column="mname" property="mname"/>
        <association property="country" javaType="Country">
            <id column="cid" property="cid"/>
            <result column="cname" property="cname"/>
        </association>
    </resultMap>

    <select id="selectMinisterById" resultMap="ministerMapper">
        select mid,mname,cid,cname
        from minister,country
        where countryId=cid and mid=#{ooo}
    </select>

</mapper>

测试类：

package com.bjpowernode.test;

public class MyTest {

    private IMinisterDao dao;
    private SqlSession sqlSession;

    @Before
    public void Before() {
        sqlSession = MyBatisUtils.getSqlSession();
        //dao:是一个动态代理
        dao = sqlSession.getMapper(IMinisterDao.class);
    }

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

    @Test
    public void test01() {
        Minister minister = dao.selectMinisterById(2);
        System.out.println(minister);
    }
}

3.2 多对一关联查询 - 通过多表单独查询方式实现

映射文件：

<?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="com.bjpowernode.dao.IMinisterDao">

    <select id="selectCountryByCountryId" resultType="Country">
        select cid,cname from country where cid=#{ooo}
    </select>

    <resultMap type="Minister" id="ministerMapper">
        <id column="mid" property="mid"/>
        <result column="mname" property="mname"/>
        <association 
            property="country" 
            javaType="Country" 
            select="selectCountryByCountryId"
            column="countryId"
        />

    </resultMap>

    <select id="selectMinisterById" resultMap="ministerMapper">
        select mid,mname,countryId from minister where mid=#{ooo}
    </select>

</mapper>