前言：
resultMap中 ：
association – 一个复杂的类型关联;许多结果将包成这种类型（一对多）
collection – 复杂类型的集合（多对一）
一、例子1:
1对多关系，1个部门拥有多个员工。 role和user是多对多关系，1个角色可能有多个员工，而1个员工也可能有多个角色。而要实现多对多，在程序中则是拆分成2个一对多。
多对一：多个user对应1个department，使用association
<?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="employee_management.mapper.UserDepartmentMapper">
<resultMap type="employee_management.entity.User"
id="userDepartmentList">
<id property="user_id" column="user_id" javaType="java.lang.Integer" />
<result property="user_name" column="user_name" javaType="java.lang.String" />
<result property="user_gender" column="user_gender" javaType="java.lang.String" />
<result property="user_email" column="user_email" javaType="java.lang.String" />
<result property="user_phone" column="user_phone" javaType="java.lang.String" />
<result property="user_address" column="user_address" javaType="java.lang.String" />
<result property="user_birthday" column="user_birthday"
javaType="java.util.Date" />
<result property="department_id" column="department_id"
javaType="java.lang.Integer" />
<!-- property是user实体类中department类属性的名字
多个user对应1个department-->
<association property="department"
javaType="employee_management.entity.Department">
<id property="department_id" column="department_id" javaType="java.lang.Integer" />
<result property="department_name" column="department_name"
javaType="java.lang.String" />
</association>
</resultMap>
<select id="getAll" resultMap="userDepartmentList">
select u.*,d.department_name from user u left join department d on u.department_id=d.department_id;
</select>
</mapper>
一对多，使用collection，1个user对应多个role：
<?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="employee_management.mapper.UserMapper">
<!-- get user list only -->
<resultMap type="employee_management.entity.User" id="userList">
<id property="user_id" column="user_id" javaType="java.lang.Integer" />
<result property="user_name" column="user_name" javaType="java.lang.String" />
<result property="user_gender" column="user_gender" javaType="java.lang.String" />
<result property="user_email" column="user_email" javaType="java.lang.String" />
<result property="user_phone" column="user_phone" javaType="java.lang.String" />
<result property="user_address" column="user_address" javaType="java.lang.String" />
<result property="user_birthday" column="user_birthday"
javaType="java.util.Date" />
<result property="department_id" column="department_id"
javaType="java.lang.Integer" />
</resultMap>
<!-- get user list with role -->
<resultMap type="employee_management.entity.User" id="userRoleList"
extends="userList">
<!-- collection中不是javaType而是ofType,通过property也知道是list，即多个。
extends属性，即在父resultMap的基础上再加上子resultMap的结果，在这里即再加上部门的信息。 -->
<collection property="roles" ofType="employee_management.entity.Role">
<id property="role_id" column="role_id" javaType="java.lang.Integer" />
<result property="role_name" column="role_name" javaType="java.lang.String" />
</collection>
</resultMap>
</resultMap>
<select id="getAll" resultMap="userList">
select * from user
</select>
<select id="getAllUserAndRole" resultMap="userRoleList">
select u.*,r.* from
user u left join user_role ur on u.user_id=ur.user_id
left join role r
on r.role_id=ur.role_id;
</select>