一对多关系，比如说根据id查询一个部门的部门信息及部门下的员工信息

在Dept类中先添加List emps属性

1、collection
DeptMapper.xml文件中

<resultMap id="deptAndEmpResultMap" type="Dept">
            <id property="did" column="did"></id>
            <result property="deptName" column="dept_name"></result>

        <!--
            collection：处理一对多的映射关系
            ofType： 标识该属性所对应的集合中存储数据的类型
         -->
            <collection property="emps" ofType="Emp">
                <id property="eid" column="eid"></id>
                <result property="empName" column="emp_name"></result>
                <result property="age" column="age"></result>
                <result property="sex" column="sex"></result>
                <result property="email" column="email"></result>
            </collection>
    </resultMap>
    <!-- Dept getDeptAndEmp(@Param("did") Integer did);-->
    <select id="getDeptAndEmp" resultMap="deptAndEmpResultMap">
       select * from t_dept left join t_emp on t_dept.did=t_emp.did where t_dept.did=#{did}
    </select>

2、分布查询
DeptMapper.xml文件中：

 <resultMap id="deptAndEmpByStepResultMap" type="Dept">
        <id property="did" column="did"></id>
        <result property="deptName" column="dept_name"></result>
        <collection property="emps"
                    select="com.atguigu.mybatis.mapper.EmpMapper.getDeptAndEmpByStepTwo"
                    column="did"
                    fetchType="eager"></collection>
    </resultMap>
    <!--Dept getDeptAndEmpByStepOne(@Param("did") Integer did);-->
    <select id="getDeptAndEmpByStepOne" resultMap="deptAndEmpByStepResultMap">
        select * from t_dept where did=#{did}
    </select>

EmpMapper.xml文件中：

 <!--List<Emp> getDeptAndEmpByStepTwo(@Param("did") Integer did);-->
    <select id="getDeptAndEmpByStepTwo" resultType="Emp">
        select * from t_emp where did=#{did}
    </select>