/**

 *

 *国家的实体类

 */

public class Country {

    private  Integer cId;    //国家的编号

    private  String cName;   //国家的名称

    //关联省会的属性

    private  Set<Provincial> provincials;

    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 Set<Provincial> getProvincials() {

        return provincials;

    }

    public void setProvincials(Set<Provincial> provincials) {

        this.provincials = provincials;

    }

    public Country(Integer cId, String cName, Set<Provincial> provincials) {

        super();

        this.cId = cId;

        this.cName = cName;

        this.provincials = provincials;

    }

    public Country() {

        super();

    }

    @Override

    public String toString() {

        return "Country [cId=" + cId + ", cName=" + cName + ", provincials="

                + provincials + "]";

    }

}

/**

 *

 *省会对应的实体类

 */

public class Provincial {

    private Integer pId;    //省会的编号

    private String pName;  //省会名称

    public Integer getpId() {

        return pId;

    }

    public void setpId(Integer pId) {

        this.pId = pId;

    }

    public String getpName() {

        return pName;

    }

    public void setpName(String pName) {

        this.pName = pName;

    }

    public Provincial(Integer pId, String pName) {

        super();

        this.pId = pId;

        this.pName = pName;

    }

    public Provincial() {

        super();

    }

    @Override

    public String toString() {

        return "Provincial [pId=" + pId + ", pName=" + pName + "]";

    }

}

public interface CountryDao {

    /**

     * 根据国家的id查询出国家的信息  以及国家下面的省会信息

     */

    Country selectCountryById(Integer cId);

}

<?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="cn.bdqn.dao.CountryDao">

<!--  这里的resultMap和之前使用的不一样，哪怕属性和字段一致 也要书写

  因为mybatis在底层封装的时候，是根据我们resultMap中写的属性来的 -->

  <resultMap type="Country" id="countryMap">

    <id property="cId" column="cid"/>

    <result property="cName" column="cname"/>

    <!-- 设置关联的集合属性 -->

     <collection property="provincials" ofType="Provincial">

      <id property="pId" column="pid"/>

      <result property="pName" column="pname"/>

     </collection>

  </resultMap>

 <!-- 这是单表的关联查询   不经常使用  因为 不能使用延迟加载 -->

    <select id="selectCountryById" resultMap="countryMap">

      select  cid,cname,pid,pname from country,provincial

      where cid=countryid and cid=#{xxx}  <!--#{xxx} 参数的占位符  -->

    </select>

</mapper>

<!-- 加载映射文件信息 -->

    <mappers>

        <mapper resource="cn/bdqn/dao/CountryMapper.xml" />

    </mappers>

public class CountryTest {

    CountryDao dao;

    SqlSession session;

    @Before

    public void before() {

        // 因为需要关闭session 需要把session提取出去

        session = SessionUtil.getSession();

        dao = session.getMapper(CountryDao.class);

    }

    @After

    public void after() {

        if (session != null) {

            session.close();

        }

    }

    /**

     * 根据国家的id查询出国家的信息  以及国家下面的省会信息

     */

    @Test

    public void test1() {

         Country country = dao.selectCountryById(1);

         System.out.println(country);

    }

}

<?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="cn.bdqn.dao.CountryDao">

    <select id="selectProvincialByCountryId" resultType="Provincial">

        select pid,pname from provincial

         where countryid=#{xxx}

         <!--#{xxx} 对应的就是resultMap中 collection节点下面的column -->

    </select>

    <resultMap type="Country" id="countryMap">

        <id property="cId" column="cid"/>

        <result property="cName" column="cname"/>

        <!--设置关联的集合属性

          select:需要关联的查询语句

         column: select关联语句中需要的参数 -->

         <collection property="provincials" ofType="Provincial"

          select="selectProvincialByCountryId"

          column="cid"/>

     </resultMap>

    <!-- 多表的查询  经常使用  可以使用延迟加载策略  -->

    <select id="selectCountryById" resultMap="countryMap">

          select  cid,cname from  country where cid=#{xxx}

          <!--#{xxx} 用户传递过来的ID  -->

    </select> 

</mapper>