引发错误，防止 MySQL 触发器中的表更新

Question

如果我在更新表之前有一个触发器，我如何抛出一个阻止更新该表的错误？

Answer 1

DELIMITER @@
DROP TRIGGER IF EXISTS trigger_name @@
CREATE TRIGGER trigger_name 
BEFORE UPDATE ON table_name
FOR EACH ROW
BEGIN

  --the condition of error is: 
  --if NEW update value of the attribute age = 1 and OLD value was 0
  --key word OLD and NEW let you distinguish between the old and new value of an attribute

   IF (NEW.state = 1 AND OLD.state = 0) THEN
       signal sqlstate '-20000' set message_text = 'hey it's an error!';     
   END IF;

END @@ 
DELIMITER ;

Answer 2

Here is one hack 可能有用。它不干净，但看起来它可能会起作用：

本质上，您只是尝试更新一个不存在的列。

Answer 3

从 MySQL 5.5 开始，您可以使用 SIGNAL syntax to throw an exception:

signal sqlstate '45000' set message_text = 'My Error Message';

状态 45000 是表示“未处理的用户定义异常”的通用状态。

---

以下是该方法的更完整示例：

delimiter //
use test//
create table trigger_test
(
    id int not null
)//
drop trigger if exists trg_trigger_test_ins //
create trigger trg_trigger_test_ins before insert on trigger_test
for each row
begin
    declare msg varchar(128);
    if new.id < 0 then
        set msg = concat('MyTriggerError: Trying to insert a negative value in trigger_test: ', cast(new.id as char));
        signal sqlstate '45000' set message_text = msg;
    end if;
end
//

delimiter ;
-- run the following as seperate statements:
insert into trigger_test values (1), (-1), (2); -- everything fails as one row is bad
select * from trigger_test;
insert into trigger_test values (1); -- succeeds as expected
insert into trigger_test values (-1); -- fails as expected
select * from trigger_test;

Answer 4

CREATE TRIGGER sample_trigger_msg 
    BEFORE INSERT
FOR EACH ROW
    BEGIN
IF(NEW.important_value) < (1*2) THEN
    DECLARE dummy INT;
    SELECT 
           Enter your Message Here!!!
 INTO dummy 
        FROM mytable
      WHERE mytable.id=new.id
END IF;
END;

Answer 5

您可以使用的另一种（hack）方法（如果您由于某种原因不在 5.5+ 上）：

如果您有必填字段，则在触发器中将必填字段设置为无效值，例如 NULL。这适用于 INSERT 和 UPDATE。请注意，如果 NULL 是必填字段的有效值（出于某种疯狂的原因），那么这种方法将不起作用。

BEGIN
    -- Force one of the following to be assigned otherwise set required field to null which will throw an error
    IF (NEW.`nullable_field_1` IS NULL AND NEW.`nullable_field_2` IS NULL) THEN
        SET NEW.`required_id_field`=NULL;
    END IF;
END

如果您使用的是 5.5+，那么您可以使用其他答案中描述的信号状态：

BEGIN
    -- Force one of the following to be assigned otherwise use signal sqlstate to throw a unique error
    IF (NEW.`nullable_field_1` IS NULL AND NEW.`nullable_field_2` IS NULL) THEN
        SIGNAL SQLSTATE '45000' set message_text='A unique identifier for nullable_field_1 OR nullable_field_2 is required!';
    END IF;
END

Answer 6

以下过程是（在 mysql5 上）抛出自定义错误并同时记录它们的方法：

create table mysql_error_generator(error_field varchar(64) unique) engine INNODB;
DELIMITER $$
CREATE PROCEDURE throwCustomError(IN errorText VARCHAR(44))
BEGIN
    DECLARE errorWithDate varchar(64);
    select concat("[",DATE_FORMAT(now(),"%Y%m%d %T"),"] ", errorText) into errorWithDate;
    INSERT IGNORE INTO mysql_error_generator(error_field) VALUES (errorWithDate);
    INSERT INTO mysql_error_generator(error_field) VALUES (errorWithDate);
END;
$$
DELIMITER ;


call throwCustomError("Custom error message with log support.");

Answer 7

不幸的是，@RuiDC 提供的答案在 MySQL 5.5 之前的版本中不起作用，因为没有为存储过程实现 SIGNAL。

我found 的解决方案是模拟信号抛出table_name doesn't exist 错误，将自定义错误消息推送到table_name。

可以使用触发器或使用存储过程来实现 hack。我将按照@RuiDC 使用的示例在下面描述这两个选项。

使用触发器

DELIMITER $$
-- before inserting new id
DROP TRIGGER IF EXISTS before_insert_id$$
CREATE TRIGGER before_insert_id
    BEFORE INSERT ON test FOR EACH ROW
    BEGIN
        -- condition to check
        IF NEW.id < 0 THEN
            -- hack to solve absence of SIGNAL/prepared statements in triggers
            UPDATE `Error: invalid_id_test` SET x=1;
        END IF;
    END$$

DELIMITER ;

使用存储过程

存储过程允许您使用动态 sql，这使得将错误生成功能封装在一个过程中成为可能。与之相对的是，我们应该控制应用程序的插入/更新方法，因此它们只使用我们的存储过程（不授予插入/更新的直接权限）。

DELIMITER $$
-- my_signal procedure
CREATE PROCEDURE `my_signal`(in_errortext VARCHAR(255))
BEGIN
    SET @sql=CONCAT('UPDATE `', in_errortext, '` SET x=1');
    PREPARE my_signal_stmt FROM @sql;
    EXECUTE my_signal_stmt;
    DEALLOCATE PREPARE my_signal_stmt;
END$$

CREATE PROCEDURE insert_test(p_id INT)
BEGIN
    IF NEW.id < 0 THEN
         CALL my_signal('Error: invalid_id_test; Id must be a positive integer');
    ELSE
        INSERT INTO test (id) VALUES (p_id);
    END IF;
END$$
DELIMITER ;