“纵许悠扬度朱户，终愁人影隔窗纱”
前言
客观世界中的对象很少有孤立存在的，例如班级，往往与班级的学生存在关联关系，如果得到某个班级的实例，那么应该可以直接获取班级对应的全部学生。反过来，如果已经得到个学生的实例，那么也应该可以访问该学生对应的班级。这种实例之间的互相访问就是关联关系。
关联关系是面向对象分析、面向对象设计最重要的知识， My Batis完全可以理解这种关联关系，如果映射得当， My Batis的关联映射将可以大大简化持久层数据的访问。
示例
这里只给出映射文件（XML文件）中的关键代码
- 一对一关系
例如：一个人对应一个身份证号
PersonMapper.xml
<?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="org.arunner.mapper.PersonMapper">
<resultMap id="personMapper" type="org.arunner.domain.Person">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="age" column="age"/>
<result property="sex" column="sex"/>
<!--一对一关联映射-->
<association property="card" column="card_id" javaType="org.arunner.domain.Card" select="org.arunner.mapper.CardMapper.selectCardById"/>
</resultMap>
<!--根据ID查询人员-->
<select id="selectPersonById" parameterType="int" resultMap="personMapper">
select * from tb_person where id = #{id} ;
</select>
</mapper>
CardMapper.xml
<?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="org.arunner.mapper.CardMapper">
<!--根据ID查询身份证信息-->
<select id="selectCardById" parameterType="int" resultType="org.arunner.domain.Card">
select * from tb_card where id = #{id} ;
</select>
</mapper>
- 一对多关系
例如：一个班级对应多个学生
ClazzMapper.xml
<?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="org.arunner.mapper.ClazzMapper">
<resultMap id="clazzResultMap" type="org.arunner.domain.Clazz">
<id property="id" column="id"/>
<result property="code" column="code"/>
<result property="name" column="name"/>
<!--班级的学生属性，因为一个班级有多个学生，所以该属性是一个集合-->
<collection property="students" javaType="ArrayList" column="id" ofType="org.arunner.domain.Student"
select="org.arunner.mapper.StudentMapper.selectStudentByClazzId" fetchType="lazy">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="age" column="age"/>
<result property="sex" column="sex"/>
</collection>
</resultMap>
<!--根据班级ID查询班级-->
<select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
select * from tb_clazz where id = #{id} ;
</select>
</mapper>
StudentMapper.xml
<?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="org.arunner.mapper.StudentMapper">
<resultMap id="studentResultMap" type="org.arunner.domain.Student">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="age" column="age"/>
<result property="sex" column="sex"/>
<!--一对一关系-->
<association property="clazz" javaType="org.arunner.domain.Clazz">
<id property="id" column="id"/>
<result property="code" column="code"/>
<result property="name" column="name"/>
</association>
</resultMap>
<!--根据班级ID查询班级-->
<select id="selectStudentByClazzId" parameterType="int" resultMap="studentResultMap">
select * from tb_student where clazz_id = #{id} ;
</select>
<!--根据ID查询学生和班级信息-->
<select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
select * from tb_student s,tb_clazz c where c.id = s.clazz_id and s.id= #{id} ;
</select>
</mapper>
- 多对多关系
例如：订单、商品和用户的关系
OrderMapper.xml
<?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">
<!-- namespace指用户自定义的命名空间。 -->
<mapper namespace="org.arunner.mapper.OrderMapper">
<resultMap type="org.arunner.domain.Order" id="orderResultMap">
<id property="id" column="oid"/>
<result property="code" column="code"/>
<result property="total" column="total"/>
<!-- 多对一关联映射:association -->
<association property="user" javaType="org.arunner.domain.User">
<id property="id" column="id"/>
<result property="username" column="username"/>
<result property="loginname" column="loginname"/>
<result property="password" column="password"/>
<result property="phone" column="phone"/>
<result property="address" column="address"/>
</association>
<!-- 多对多映射的关键:collection -->
<collection property="articles" javaType="ArrayList"
column="oid" ofType="org.arunner.domain.Article"
select="org.arunner.mapper.ArticleMapper.selectArticleByOrderId"
fetchType="lazy">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="price" column="price"/>
<result property="remark" column="remark"/>
</collection>
</resultMap>
<!-- 注意，如果查询出来的列同名，例如tb_user表的id和tb_order表的id都是id，同名，需要使用别名区分 -->
<select id="selectOrderById" parameterType="int" resultMap="orderResultMap">
SELECT u.*,o.id AS oid,CODE,total,user_id
FROM tb_user u,tb_order o
WHERE u.id = o.user_id
AND o.id = #{id}
</select>
<!-- 根据userid查询订单 -->
<select id="selectOrderByUserId" parameterType="int" resultType="org.arunner.domain.Order">
SELECT * FROM tb_order WHERE user_id = #{id}
</select>
</mapper>
UserMapper.xml
<?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">
<!-- namespace指用户自定义的命名空间。 -->
<mapper namespace="org.arunner.mapper.UserMapper">
<resultMap type="org.arunner.domain.User" id="userResultMap">
<id property="id" column="id"/>
<result property="username" column="username"/>
<result property="loginname" column="loginname"/>
<result property="password" column="password"/>
<result property="phone" column="phone"/>
<result property="address" column="address"/>
<!-- 一对多关联映射:collection -->
<collection property="orders" javaType="ArrayList"
column="id" ofType="org.arunner.domain.User"
select="org.arunner.mapper.OrderMapper.selectOrderByUserId"
fetchType="lazy">
<id property="id" column="id"/>
<result property="code" column="code"/>
<result property="total" column="total"/>
</collection>
</resultMap>
<select id="selectUserById" parameterType="int" resultMap="userResultMap">
SELECT * FROM tb_user WHERE id = #{id}
</select>
</mapper>
ArticleMapper.xml
<?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">
<!-- namespace指用户自定义的命名空间。 -->
<mapper namespace="org.arunner.mapper.ArticleMapper">
<select id="selectArticleByOrderId" parameterType="int"
resultType="org.arunner.domain.Article">
SELECT * FROM tb_article WHERE id IN (
SELECT article_id FROM tb_item WHERE order_id = #{id}
)
</select>
</mapper>