Update inventory after transaction - mysql

Question

CREATE TABLE IF NOT EXISTS `inventory` (
`id_product` INT(4) ZEROFILL NOT NULL AUTO_INCREMENT,
`model` varchar(45) NOT NULL,
`price_new` FLOAT DEFAULT '0',
sell_price float,
`condition1` varchar(45) NOT NULL,
`launch_date` DATE NOT NULL,
`stock` INT DEFAULT '0',
PRIMARY KEY (`id_product`));
CREATE TABLE IF NOT EXISTS `order` (
`id_order` INT(4) ZEROFILL NOT NULL AUTO_INCREMENT,
`id_customer` INT(6) ZEROFILL NOT NULL,
`subtotal` FLOAT DEFAULT '0',
`discount` float DEFAULT '0',
`tax_rate` float DEFAULT '0.23',
`total` float DEFAULT '0',
`date` DATETIME DEFAULT NOW(),
PRIMARY KEY (`id_order`),
CONSTRAINT `fk_order_1`
FOREIGN KEY (`id_customer`)
REFERENCES `iSAVE`.`customer` (`id_customer`)
ON DELETE RESTRICT
ON UPDATE CASCADE);
CREATE TABLE IF NOT EXISTS `transaction` (
`id_transaction` INT(4) ZEROFILL NOT NULL AUTO_INCREMENT PRIMARY KEY,
`id_order`INT(4) ZEROFILL NOT NULL,
`status` char(20) default 'Not completed',
FOREIGN KEY (`id_order`) REFERENCES `iSAVE`.`order` (`id_order`)
);
CREATE TABLE IF NOT EXISTS `order_item` (
`id_order_item` INT(4) ZEROFILL NOT NULL AUTO_INCREMENT PRIMARY KEY,
`id_product` INT(4) ZEROFILL NOT NULL,
`id_order` INT(4) ZEROFILL NOT NULL,
`quantity` INT DEFAULT NULL,
FOREIGN KEY (`id_product`) REFERENCES `iSAVE`.`inventory` (`id_product`),
FOREIGN KEY (`id_order`) REFERENCES `iSAVE`.`order` (`id_order`)
);
I have those tables and my objective is after i update transactions to completed it goes to my inventory table and update the stock. The items that it should update are the items that i choose in order items.
For now i have this peace of code but its not working ...
delimiter $
create trigger update_stock
after update on `transaction`
for each row
begin
if new.`status` = 'Completed' THEN
update inventory
set inventory.stock = inventory.stock - new.quantity
where inventory.id_product = new.id_product;
end if;
End $$
delimiter ;
Edited trigger:
delimiter $$
CREATE TRIGGER trigg1 after update on `transaction` for each row
BEGIN
UPDATE `inventory`
SET stock = (SELECT stock - new.quantity FROM order_item WHERE order_item.id_product = inventory.id_product limit 1);
end $$
delimiter ;

Answer 1

You seem to need an extra join to get the quantity from order_item:
update inventory i join
order_item oi
using (id_product)
set i.stock = i.stock - oi.quantity
where oi.id_order = new.id_order;