外键肯定定义在多方表，有外键肯定充当在多方的。
3.1.1 一对多关联查询
一方可以看见多方，这里的一对多查询是指，在查询一方对象的时候，同时也将其关联的多方对象也都查询出来。
方式一：多表连接查询
<?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.ICountryDao">
<resultMap type="Country" id="countryMapper">
<id column="cid" property="cid"/>
<result column="cname" property="cname"/>
<collection property="minsters" ofType="Minister">
<id column="mid" property="mid"/>
<result column="mname" property="mname"/>
</collection>
</resultMap>
<select id="selectCountryById" resultMap="countryMapper">
select cid,cname,mid,mname
from country,minister
where cid = countryId
and cid = #{id}
</select>
</mapper>方式二：多表单独查询方式（用的比较多，延迟查询）
<?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.IStudentDao">
<select id="selectMinisterByCountry" resultType="Minister">
select mid,mname by minister where mid = #{cid}
</select>
<resultMap type="Country" id="CountryMapper">
<id column="cid" property="cid"/>
<result column="cname" property="cname"/>
<collection property="ministers"
ofType="Minister"
select="selectMinisterByCountry"
column="cid"/>
</resultMap>
<select id="selectCountryById" resultMap="CountryMapper">
select cid,cname from country where cid = 2
</select>
</mapper>
<resultMap>:映射信息
type:所选的类名（一对多中一代表的类名）
<id>: 代表主键
column：表中字段
property：类中属性
<result> 其他键（属性）
<collection> 一对多中的多
property：代表一对多中的多的属性
ofType：集合 集合成的类名
select ：单独查询的另一个选择
column：两个单独查询的相交点（关联属性）
3.1.2 多对一查询
方式一：多表连接查询
<?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 cid = countryId
and mid = #{mid}
</select>
</mapper>
多对一的<association> 类比一对多的<collection>
<assciation property="多对一中一的属性"，javatype=“l类比ofType 属性对应的类”/>
方式二：多表单独查询
<?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="selectCountryById" resultMap="Country">
select cid,cname from country where cid = #{countryId}
</select>
<resultMap type="minister" id="ministerMapper">
<id column="mid" property="mid"/>
<result column="mname" property="mname"/>
<association property="country"
javaType="Country"
select="selectCountryById"
column="countryId"
/>
</resultMap>
<select id="selectMinisterById" resultMap="ministerMapper">
select mid,mname
from minister
where mid = #{mid}
</select>
</mapper>关键是column=“countryId” 找到两个查询之间的关联点。
3.1.3 自关联查询
（1）以一对多方式处理
A、查询指定栏目的所有子孙栏目
递归调用
<?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.INewsLabelDao">
<resultMap type="Newslabel" id="newsLabelMapper">
<id column="id" property="id"/>
<result column="name" property="name"/>
<collection property="children"
ofType="NewsLabel"
select="selectChildrenByParent"
column="id"/>
</resultMap>
<select id="selectChildrenByParent" resultMap="newslabelMapper">
select id,name from newslabel where pid=#{id}
</select>
</mapper>
B、查询指定栏目及其所有子孙栏目
<?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.INewsLabelDao">
<select id="selectNewslabelByParent" resultMap="newsLabelMapper">
select id,name from newslabel where pid = #{id}
</select>
<resultMap type="Newslabel" id="newsLabelMapper">
<id column="id" property="id"/>
<result column="name" property="name"/>
<collection property="children"
ofType="NewsLabel"
select="selectNewslabelByParent"
column="id"/>
</resultMap>
<select id="selectNewsLabelById" resultMap="newslabelMapper">
select id,name from newslabel where id=#{id}
</select>
</mapper>
(2)以多对一方式来处理
<?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.INewsLabelDao">
<resultMap type="NewsLabel" id="newslabelMapping">
<id column="id" property="id"/>
<result column="name" property="name"/>
<association property="parent"
javaType="NewsLabel"
select ="selectNewsLabelById"
column="pid"/>
</resultMap>
<select id="selectNewsLabelById" resultMap="newslabelMapping">
select id,name,pid from newslabel where id = #{pid}
</select>
</mapper>
3.1.4 多对多关联查询
两个一对多构成的
中间件middle为多，student和course为一
<?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.IStudentDao">
<resultMap type="Student" id="studentMapping">
<id column="sid" property="sid"/>
<result column="sname" property="sname"/>
<collection property="courses" ofType="Course">
<id column="cid" property="cid"/>
<result column="cname" property="cname"/>
</collection>
</resultMap>
<select id="selectNewsLabelById" resultMap="studentMapping">
select sid,sname,cid,cname
from student,middle,course
where sid = studentId and cid = courseId and sid=#{sid}
</select>
</mapper>多对多转换为一对多，采用多表连接查询方式解决。