How can i Update inventory using TRIGGER - mysql

Question

Well here is the problem...I want to update the inventory table at my database which includes
Sale(ProductID,Quantity,Price)
Invlevel(ProductID,Quantity)
I want to use triggers for this update. I have to use 2 triggers (I think). 1 at the sale tables which will be something like
CREATE TRIGGER `sale_AINS` AFTER INSERT ON sale FOR EACH ROW
begin
update Invlevel set quantity = Invlevel.quantity-"sale.quantity" where
Invlevel.ProductID = "sale.ProductID";
END
with this trigger, when a sale comes up, I want to adjust the inventory level at the product which the sale came to. However, even though this trigger has no errors and runs properly when I insert into sale table, it doesn't make any changes at the Invlevel table and I don't know why ( :# ).
Moreover, I made the trigger at the Invlevel table like that
CREATE TRIGGER `invlevel_AUPD` after UPDATE on invlevel FOR EACH ROW
if invlevel.quantity < 20
begin
insert into orders
(select productid,amount from reorder where reorder.productid = invlevel.productid )
end
*(orders,reorder are other tables)
It says to me I have syntax error which I don't know what it is and also even if I find it I can't even check the logic because the other trigger isn't even working... Any thought or help about it would be really helpful.

Answer 1

Try
First trigger
CREATE TRIGGER sale_AINS
AFTER INSERT ON sale
FOR EACH ROW
UPDATE invlevel
SET quantity = quantity - NEW.quantity
WHERE productid = NEW.productid;
In this trigger you don't need BEGIN END block since there is only one statement. In order to access column values or a row(s) that being inserted you need to use NEW keyword.
And the second one
DELIMITER $$
CREATE TRIGGER invlevel_AUPD
AFTER UPDATE ON invlevel
FOR EACH ROW
BEGIN
IF NEW.quantity < 20 THEN
INSERT INTO orders (productid, amount)
SELECT productid, amount
FROM reorder
WHERE productid = NEW.productid;
END IF;
END$$
DELIMITER ;
In the second trigger IF statement was out of BEGIN END block and syntax for IF was wrong. It should be IF ... THEN ... END IF;. And the same problems with a NEW keyword.
Here is SQLFiddle demo
You can put your IF condition into WHERE clause and make it more succinct like this
CREATE TRIGGER invlevel_AUPD
AFTER UPDATE ON invlevel
FOR EACH ROW
INSERT INTO orders (productid, amount)
SELECT productid, amount
FROM reorder
WHERE productid = NEW.productid
AND NEW.quantity < 20;
Here is updated SQLFiddle demo