1、查询各job_id的员工工资的最大值，最小值，平均值，总和，并按job_id升序排序

SELECT MAX(salary),MIN(salary),AVG(salary),SUM(salary),job_id

FROM employees

GROUP BY job_id

ORDER BY job_id ASC;

2、查询员工最高工资和最低工资的差距（DIFFERENCE）

SELECT MAX(salary)-MIN(salary) DIFFERENCE

FROM employees

ORDER BY job_id;

3、查询各个管理者手下员工的最低工资，其中最低工资不能低于6000，没有管理者的员工不计算在内

SELECT MIN(salary),manager_id

FROM employees

WHERE manager_id IS NOT NULL

GROUP BY manager_id

HAVING MIN(salary)>=6000;

4、查询所有部门的编号，员工数量和工资平均值，并按平均工资降序

SELECT department_id, COUNT(*),AVG(salary) a

FROM employees

GROUP BY department_id

ORDER BY a DESC;

5、选择具有各个job_id的员工人数

SELECT COUNT(*) 个数 ,job_id

FROM employees

GROUP BY job_id;