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排序查询
order by [asc(升序，默认的)|desc（降序）]
order by 支持单个字段，多个字段，表达式，函数，别名
放在sql语句的最后面（limit 子句 除外）；
SELECT * FROM employees ORDER BY salary;
SELECT * FROM employees ORDER BY salary DESC;
SELECT * FROM employees WHERE department_id >=90 ORDER BY hiredate DESC;
# 按年薪排序
SELECT * ,salary*12*(1+IFNULL(commission_pct,0)) nx
FROM employees
ORDER BY nx DESC;
# 更last_name 的长度排序(要使用到一个函数Length（）)
SELECT * FROM employees ORDER BY LENGTH(last_name) DESC;
# 查询员工信息，先按工资排序，在按员工编号排序；
# (其实就是工资相等排不了，在按员工编号排序)
SELECT * FROM employees ORDER BY salary DESC,employee_id DESC;