1. 查询个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

select max(salary),min(salary),avg(salary),sum(salary),job_id from employees
group by job_id
order by job_id;

2. 查询员工最高工资和最低工资的差距

select max(salary)-min(salary) difference from employees;

3. 查询各个管理者手下的员工的最低工资，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. 查询所有的部门编号，员工数量和工资平均值，并按平均公司降序

select department_id, count(*),avg(salary) avg
from employees
group by department_id
order by avg desc;

5. 选择各个job_id 的员工个数

select count(*),job_id
from employees
group by job_id;