/***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 Country(Integer cId, String cName) {super();this.cId = cId;this.cName = cName;}public Country() {super();}@Overridepublic String toString() {return "Country [cId=" + cId + ", cName=" + cName ;}}

/*** *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//关联的国家属性private  Country country;public Country getCountry() {return country;}public void setCountry(Country country) {this.country = country;}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();}@Overridepublic String toString() {return "Provincial [pId=" + pId + ", pName=" + pName + ", country="+ country + "]";}}

public interface ProvincialDao {/*** 根据省会的id查询出省会和对应国家的信息  */Provincial selectProvincialById(Integer pId);
}

<?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.ProvincialDao"><!-- 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="Provincial" id="provincialMap"><id property="pId" column="pid"/><result property="pName" column="pname"/><!--  设置关联的属性 --><association property="country" javaType="Country"><id property="cId" column="cid"/><result property="cName" column="cname"/></association></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="selectProvincialById" resultMap="provincialMap">select  cid,cname,pid,pname from country,provincialwhere cid=countryid and pid=#{xxx}   <!--  #{xxx} 参数的占位符  --></select>  </mapper>

    <!-- 加载映射文件信息 --><mappers><mapper resource="cn/bdqn/dao/ProvincialMapper.xml" /></mappers>

public class ProvincialTest {ProvincialDao dao;SqlSession session;@Beforepublic void before() {// 因为需要关闭session 需要把session提取出去session = SessionUtil.getSession();dao = session.getMapper(ProvincialDao.class);}@Afterpublic void after() {if (session != null) {session.close();}}/*** 根据省会的id查询出省会和对应国家的信息  */@Testpublic void test1() {Provincial provincial = dao.selectProvincialById(1);System.out.println(provincial);}}

<?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.ProvincialDao"><select id="selectCountryByProvincialId" resultType="Country">select cid,cname from country where cid=#{xxx}<!--#{xxx}就是resultMap 中 association节点中的column属性 --></select><resultMap type="Provincial" id="provincialMap"><id property="pId" column="pid"/><result property="pName" column="pname"/><!-- 设置关联的属性     select：关联的查询结果 --><association property="country" javaType="Country"select="selectCountryByProvincialId" column="countryid" /></resultMap><!-- 多表的单独查询   常用的方式 可以使用延迟加载策略 --><select id="selectProvincialById" resultMap="provincialMap">select  pid,pname,countryid from  provincialwhere  pid=#{xxx}   <!--  #{xxx} 用户传递参数的占位符  --></select></mapper>