aS在数据库中后面可以起到一个别名的作用，这样可以方便给开发的人员的理解
select
username as 账号 ,
password as 密码,
mingzi as 名字,
zhengjianhao as 证件号,
dianhua as 电话,
zhuceriqi as 注册日期,
zhuangtai as 状态,
quanxian as 权限,
shengyutianshu as 剩余天数
from user
引用作者：https://www.cnblogs.com/zhaotiancheng/p/6692553.html
原文：https://www.cnblogs.com/liuchunyong/p/11263023.html