学生选课系统，实现学生可以选择多门选课课程，并将有效选课结果保存到数据库。学生需要登录后，才能选课。让学生可以在选课系统通过多种方式查询到要选的课程信息。

选课规则：1、每个学生可以选多门课程，多次选课的总学分不能超过6学分；2、不能重复选择一门课程；3、每一门课程的选课人数都有数量限制，当某门课程的选课名额满时，则应另选课程。4、凭用户名和密码登录，通过提交某一课程号来选课-Student enrollment system, students can choose elective courses more doors, and save the results to a　effectively elective. Students need to log in to enrollment. So that students can query the system in elective courses to choose the information through a variety of ways.

Course Selection Rules: 1. Each student can choose more courses, numerous elective total credits can not exceed 6 credits　2, choose a course can not be repeated　3, the number of elective courses each have to limit the number, when a when elective courses quota is full, you should choose another course. 4, with the user name and password, through the submission of a number to elective courses