Add the correct MySQL trigger - mysql

Question

I have a trigger that should work, but it does not work.
BEGIN
DECLARE `refresh` VARCHAR(256);
SET `refresh` = concat((SELECT `refresh` FROM `mobile_app_drivers` WHERE `id` = 1), '&');
UPDATE `mobile_app_drivers` SET `refresh` = #refresh WHERE `id` = 1;
END
When it is executed, the cell **refresh** still broadcasts an empty string.
// HOW I CREATE IT
DROP TRIGGER IF EXISTS `SET_UNIQUE_KEY`;
CREATE DEFINER=`root`#`localhost` TRIGGER `SET_UNIQUE_KEY` AFTER UPDATE ON `mobile_app_orders` FOR EACH ROW
BEGIN
DECLARE `refresh` VARCHAR(256);
SET `refresh` = concat((SELECT MAX(`refresh`) FROM `mobile_app_drivers` WHERE `id` = 1), '&');
UPDATE `mobile_app_drivers` SET `refresh` = #refresh WHERE `id` = NEW.driver;
END
I need, when any record from the table is updated, then a new substring was added from the value refresh from the table mobile_app_drivers.
Thank you!

Answer 1

You are using session variable #refresh. However, you have not defined it anywhere. So it will considered empty only.
I modified your trigger to make it more clear and logical to follow. Declared a new variable new_refresh and we can set its value using Into clause inside a Select statement.
Now, call the update statement.
Also, you need to redefine Delimiter to something else (eg: $$), instead of (;).
At the end, redefine the DELIMITER to ;
You can modify your Create Trigger statement to as follows:
DELIMITER $$
DROP TRIGGER IF EXISTS `SET_UNIQUE_KEY` $$
CREATE DEFINER=`root`#`localhost` TRIGGER `SET_UNIQUE_KEY` AFTER UPDATE ON `mobile_app_orders` FOR EACH ROW
BEGIN
DECLARE new_refresh VARCHAR(256) DEFAULT '';
SELECT CONCAT(MAX(`refresh`), '&') INTO `new_refresh`
FROM `mobile_app_drivers`
WHERE `id` = 1;
UPDATE `mobile_app_drivers` SET `refresh` = new_refresh WHERE `id` = NEW.driver;
END $$
DELIMITER ;

Answer 2

Why are you using a variable? Just do:
DROP TRIGGER IF EXISTS `SET_UNIQUE_KEY`;
DELIMITER $$
CREATE DEFINER=`root`#`localhost` TRIGGER `SET_UNIQUE_KEY` AFTER UPDATE ON `mobile_app_orders`
FOR EACH ROW
BEGIN
UPDATE mobile_app_drivers CROSS JOIN
(SELECT MAX(refresh) as max_refresh
FROM mobile_app_drivers
WHERE id = 1
) mad1
SET refresh = concat(mad1.max_refresh, '&')
WHERE id = NEW.driver;
END;
DELIMITER ;