分步查询

<!-- 部门 Mapper XML 文件 --><!-- 查询某个部门及其下的所有员工，使用 resultMap 定义嵌套查询 -->
<select id="selectDepartmentWithEmployees" resultMap="DepartmentWithEmployeesResultMap">SELECT * FROM department WHERE department_id = #{departmentId}
</select><resultMap id="DepartmentWithEmployeesResultMap" type="Department"><!-- 部门的属性映射 --><id property="departmentId" column="department_id"/><result property="departmentName" column="department_name"/><!-- 嵌套查询，查询该部门下的所有员工 --><collection property="employees" ofType="Employee" column="department_id" select="selectEmployeesByDepartmentId"/>
</resultMap>

<!-- 员工 Mapper XML 文件 --><!-- 查询某个部门下的所有员工 -->
<select id="selectEmployeesByDepartmentId" resultType="Employee">SELECT * FROM employee WHERE department_id = #{departmentId}
</select>

级联查询

<!-- 部门 Mapper XML 文件 --><!-- 查询某个部门及其下的所有员工，使用级联查询 -->
<select id="selectDepartmentWithEmployees" resultMap="DepartmentWithEmployeesResultMap">SELECT d.*, e.* FROM department dLEFT JOIN employee e ON d.department_id = e.department_idWHERE d.department_id = #{departmentId}
</select><resultMap id="DepartmentWithEmployeesResultMap" type="Department"><id property="departmentId" column="department_id"/><result property="departmentName" column="department_name"/><!-- 嵌套结果集映射，将员工的属性映射到 employees 集合中 --><collection property="employees" ofType="Employee"><id property="employeeId" column="employee_id"/><result property="employeeName" column="employee_name"/><!-- 其他员工的属性映射 --></collection>
</resultMap>