选课系统文档，里面内容自己看啦！包括很多类图什么的-Elective system documentation, which the content of their own看啦! Including many of what type of graph

主要描述学生选课系统数据库的整体设计，包括背景、意义、数据库相关知识等-Mainly describes the overall design of the student elective system database, including the background, significance, database knowledge