GROUP BY 跟 ORDER BY一起使用

正确使用如下：

SELECT dep.department_name, MAX(emp.salary), MIN(emp.salary)  FROM departments dep  LEFT JOIN employees emp    ON (dep.department_id = emp.department_id) GROUP BY dep.department_name--order by 的列，必须是出现在group by 子句里的列 ORDER BY dep.department_name DESC 

错误使用如下：

（执行后报错：ORA-00979：不是GROUP BY 表达式）

SELECT dep.department_name, MAX(emp.salary), MIN(emp.salary)  FROM departments dep  LEFT JOIN employees emp    ON (dep.department_id = emp.department_id) GROUP BY dep.department_name--order by 的列，应该出现在group by 子句中 ORDER BY dep.department_id DESC