在 MySQL 触发器中抛出阻止表更新的错误

下面是一个可行的方法，虽然不干净，但看起来可行:

在 MySQL 5.5中，您可以使用 引发异常的 SIGNAL语法:

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。

这里有一个更完整的例子:

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;

不幸的是,@RuiDC 提供的答案在5.5之前的 MySQL 版本中不起作用，因为没有针对存储过程的 信号实现。

我使用的 找到了解决方案是模拟抛出 table_name doesn't exist错误的信号，将定制的错误消息推送到 table_name。

可以使用触发器或存储过程实现黑客攻击。下面我将按照@RuiDC 使用的示例描述这两个选项。

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，这使得在一个过程中封装错误生成功能成为可能。相反，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 INSERT/UPDATE 直接特权)。

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 ;

下面的过程是(在 mysql5上)抛出自定义错误并同时记录它们的一种方法:

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.");

另一个你可以使用的方法(如果你不是5.5 + 的话) :

如果有必填字段，则在触发器中将必填字段设置为无效值，如 NULL。这对 INSERT 和 UPDATE 都适用。请注意，如果 NULL 是所需字段的有效值(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

如果你是5.5 + ，那么你可以使用信号状态，如其他答案所述:

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

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;

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 ;