排序查询
语法:

select 查询字段1
from 表
where 【筛选条件】
order by 要排序字段2 asc升序、desc 降序，要排字段3 asc升序、desc降序
如果不写默认为升序

案例:查询员工信息，要求工资从高到低排序

SELECT 
    *
FROM 
    employees
ORDER BY salary DESC;

#案例2：查询部门编号>=90，按入职日期升序的员工信息

SELECT
    *
FROM
    employees
WHERE
    department_id >=90
ORDER BY hiredate ASC;

#案例：按表达式排序，如按年薪高低显示年薪

SELECT
    salary*12*(1+IFNULL(commission_pct,0))
FROM
    employees
ORDER BY salary*12*(1+IFNULL(commission_pct,0)) DESC;

#案例:将排序字段起别名，再将按别名排序

SELECT
    salary*12*(1+IFNULL(commission_pct,0)) AS 年薪
FROM 
    employees
ORDER BY 
    年薪 ASC;

#案例:按姓名的长度降序显示员工的姓名和工资（按函数排序）
SELECT LENGTH('aaaa'); #显示长度为4

SELECT
    LENGTH(last_name) AS 字节长度,
    last_name,
    salary
FROM 
    employees
ORDER BY 
    LENGTH(last_name) DESC;

#案例:查询员工信息，要求先按工资排序，再按员工编号排序,(按多个字段排序)

SELECT
    *
FROM
    employees
ORDER BY
    salary ASC,employee_id DESC;