MySQL触发器未正确搜查变量

我是MySQL触发器的新手,而且假如条目无效,则想防备条目存储在数据库中.在这种环境下,我想搜查模块年份是否大于课程中的年份.

这是我的进程(此进程在插入之前执行).

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;

为什么这应承任何课程年份入学,我该怎样办理？ 最佳谜底 编辑：修改后的谜底,此刻我更好地领略了题目.

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

可是,假如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)

缘故起因是触发器中的变量为NULL,因此NEW.course_year> num_years不正确,不会激发非常.

要办理此题目,请搜查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

假如您实举动找不到的course_id插入模块,这也会激发错误.同样,这将使num_years为NULL,因此我们必要在触发器中举办搜查.

insert into modules set module_id=2,course_id=5,course_year=99;
ERROR 1644 (45000): course_year is larger than the course length

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