如果我trigger
before the update
在表上有,如何抛出错误以阻止对该表进行更新?
Answers:
从MySQL 5.5开始,您可以使用SIGNAL
语法引发异常:
signal sqlstate '45000' set message_text = 'My Error Message';
状态45000是表示“未处理的用户定义的异常”的一般状态。
这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;
不幸的是,@ RuiDC提供的答案在5.5之前的MySQL版本中不起作用,因为没有实现SIGNAL针对存储过程实现。
我发现的解决方案是模拟引发table_name doesn't exist
错误的信号,将自定义的错误消息推送到table_name
。
黑客可以使用触发器或存储过程来实现。我将在@RuiDC使用的示例之后,在下面介绍这两个选项。
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$$
DELIMITER ;
存储过程允许您使用动态sql,这使得将错误生成功能封装在一个过程中成为可能。反对的是,我们应该控制应用程序的插入/更新方法,因此它们仅使用我们的存储过程(不向INSERT / UPDATE授予直接特权)。
DELIMITER $$
-- 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 ;
以下过程是(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.");
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;
可以使用的另一种(hack)方法(如果您由于某种原因未使用5.5+):
如果您具有必填字段,则在触发器内,将必填字段设置为无效值,例如NULL。这对于INSERT和UPDATE均适用。请注意,如果NULL是必填字段的有效值(出于某种疯狂的原因),则此方法将不起作用。
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+,则可以使用其他答案中所述的信号状态:
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
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 ;