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>