学生选课系统库表设计
背景——用例图：一个基础的学生选课系统
ER图设计如下：(学生和课程是n - m的关系，可修改的原图找不到了，悉知)
库表设计：
---专业表
CREATE TABLE major(
major_id INT PRIMARY KEY AUTO_INCREMENT,
major_name VARCHAR(20)
);
---学生表
CREATE TABLE student(
student_id INT PRIMARY KEY AUTO_INCREMENT,
student_name VARCHAR(20),
grade VARCHAR(10),
pwd VARCHAR(20),
major_id INT,
CONSTRAINT FOREIGN KEY(major_id) REFERENCES major(major_id) ON DELETE CASCADE
);
---教师表
CREATE TABLE teacher(
teacher_id INT PRIMARY KEY AUTO_INCREMENT,
teacher_name VARCHAR(20),
major_id INT,
grade VARCHAR(10),
pwd VARCHAR(20),
CONSTRAINT FOREIGN KEY(major_id) REFERENCES major(major_id) ON DELETE CASCADE
);
---课程表
CREATE TABLE course(
course_id INT PRIMARY KEY AUTO_INCREMENT,
course_name VARCHAR(20),
grade VARCHAR(10),
start int,
end int,
deadline DATE,
teacher_id INT,
stu_number int,
CONSTRAINT FOREIGN KEY (teacher_id) REFERENCES teacher(teacher_id) ON DELETE CASCADE
);
---选课表
CREATE TABLE choose(
student_id INT,
course_id INT,
score INT,
CONSTRAINT FOREIGN KEY (student_id) REFERENCES student(student_id) ON DELETE CASCADE,
CONSTRAINT FOREIGN KEY (course_id) REFERENCES course(course_id) ON DELETE CASCADE
);
触发器设计：
/*
----当学生选择课程时，自动给该课程的人数加1
名称： incre_trigger_SC
*/
create trigger incre_trigger_SC on choose for insert
as
declare @num int;
declare @teacher_id char(12);
declare @courseno char(4);
set @courseno = (select course_id from inserted);
set @num = 0;
set @num =(select stu_number from course where course_id = @courseno)+1;
update course set stu_number=@num where course_id = @courseno ;
/*
----当学生退选课程时，自动给该课程的人数减1
名称： decre_trigger_SC
*/
create trigger decre_trigger_SC on choose for delete
as
declare @num int;
declare @teacherno char(12);
declare @courseno char(4);
set @teacherno = (select teacher_id from inserted);
set @courseno = (select course_id from inserted);
set @num = 0;
set @num =(select stu_number from course where course_id = @courseno)-1;
update course set stu_number=@num where course_id = @courseno;