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

    1. select job_id,max(salary),min(salary),sum(salary),avg(salary) 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（DIFFERENCE）

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

    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(1) 个数,job_id from employees group by job_id;