说明：  本帮助文档是从我的软件工程大作业——双选信息管理系统上取下来的，我重新整理了一下，把SQL语言操作的大部分内容都包括进去了，因此把它作为手头一个‘速查手册’是挺不错的！
(This help text is from my software engineering major operation- Dual-option information management system to take down, I refresh the look, the SQL language most of operations are included, so do it as a ' speed check manual ' is very good!)