SQL duplicate trigger - mysql

Question

I have simple project for database classes there i have shop database which implement such logic: for each customer in order table we have order with type = 'CART' when customer confirm his order his order change it type to 'ORDER' and we create a new CART for our customer.
Now i want to write trigger that allows me to control that each customer has only one CART.
I write something like this
DELIMITER $$
USE `newshop`$$
CREATE
DEFINER=`root`#`localhost`
TRIGGER `newshop`.`cart_check`
BEFORE INSERT ON `newshop`.`order`
FOR EACH ROW
BEGIN
DECLARE msg VARCHAR(255);
DECLARE count_pn INTEGER;
SELECT count(*) INTO count_pn FROM newshop.order
where newshop.order.type = NEW.type
and NEW.user_id = newshop.order.user_id
and NEW.type = 'CART'
or NEW.type = 'cart'
LIMIT 1;
if count_pn > 0 THEN
BEGIN
set msg = 'Oh no';
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = msg;
end;
ELSE
BEGIN
set msg = 'Oh yeah';
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = msg;
END;
END IF;
END$$
then when i try to insert into table new order with type cart for user with id = 1, that already has a cart - this trigger doesn't allow me to do this and if i try to add a cart for another user, that doesn't have cart - this trigger also doesn't allow me to do this.

Answer 1

In else, you also have the SIGNAL SQLSTATE=45000, which is unhandled exception, it will cause return as failed.
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = msg;