我是MySQL触发器的新手,并且如果条目无效,则想防止条目存储在数据库中.在这种情况下,我想检查模块年份是否大于课程中的年份.
这是我的过程(此过程在插入之前执行).
SELECT num_of_years INTO @num_years FROM courses WHERE courses.course_id = NEW.course_id;
IF NEW.course_year > @num_years THEN
UPDATE `Error: invalid_id_test` SET x=1;
END;
为什么这允许任何课程年份入学,我该如何解决？
最佳答案
编辑：修改后的答案,现在我更好地理解了问题.
这是一个测试.
create table courses (
course_id int primary key,num_of_years tinyint unsigned default 1
);
create table modules (
module_id int primary key,course_id int,course_year tinyint unsigned
);
delimiter ;;
create trigger t before insert on modules for each row
begin
declare num_years tinyint unsigned;
select num_of_years into num_years from courses where course_id = NEW.course_id;
if NEW.course_year > num_years then
signal sqlstate '45000'
set message_text = 'course_year is larger than the course length';
end if;
end;;
delimiter ;
这种作品：
insert into courses set course_id=1,num_of_years=3;
insert into modules values set module_id=1,course_id1,course_year=4;
ERROR 1644 (45000): course_year is larger than the course length
但是,如果courses.num_of_years为NULL,则不会阻止INSERT.
insert into courses set course_id=2,num_of_years=NULL;
insert into modules set module_id=2,course_id=2,course_year=99;
Query OK,1 row affected (0.01 sec)
原因是触发器中的变量为NULL,因此NEW.course_year> num_years不正确,不会引发异常.
要解决此问题,请检查NULL.
delimiter ;;
create trigger t before insert on modules for each row
begin
declare num_years tinyint unsigned;
select num_of_years into num_years from courses where course_id = NEW.course_id;
if num_years is NULL or NEW.course_year > num_years then
signal sqlstate '45000'
set message_text = 'course_year is larger than the course length';
end if;
end;;
delimiter ;
insert into modules set module_id=2,course_year=99;
ERROR 1644 (45000): course_year is larger than the course length
如果您尝试为找不到的course_id插入模块,这也会引发错误.同样,这将使num_years为NULL,因此我们需要在触发器中进行检查.
insert into modules set module_id=2,course_id=5,course_year=99;
ERROR 1644 (45000): course_year is larger than the course length