Mysql trigger is not updating record - mysql

Question

I have two tables "Products" and "stocks". Products table will have all the products. In stock table, I save stock of product as a new entry each time.
I have two type of products. Product of "1" type will have stock as a number and product of "0" type will have stock as weight. Therefore, in products table I have created two columns "stock_weight (to keep total weight entered in stock table)" and "pieces (to keep total quantity entered in stock table)".
For this I have created a trigger on stock table that will execute to update total weight or quantity(pieces) in products table whenever I will insert new record into this table. Below is the code of trigger:
BEGIN
DECLARE product_type tinyint default 0;
SET product_type = (Select product_type from products where id = NEW.product_id);
IF (product_type = 0) THEN
UPDATE products set stock_weight = stock_weight + NEW.net_weight where id = NEW.product_id;
ELSE
UPDATE products set stock_pieces = stock_pieces + NEW.pieces where id = NEW.product_id;
END IF;
END
But after inserting new record in stock table for any product, nothing is updating in product table. I have debugged trigger and trigger is executing but nothing is updating in product table.
Can someone please tell me what I am missing and what thing I am doing wrong?

Answer 1

Don't give variables the same name as columns and anticipate nulls
DROP TABLE IF EXISTS PRODUCTS,STOCKS;
create table products(id int,product_type int,stock_weight int,stock_pieces int);
create table stocks(product_id int,net_weight int,pieces int);
drop trigger if exists t;
delimiter $$
create trigger t after insert on stocks
for each row
BEGIN
DECLARE vproduct_type tinyint default 0;
SET vproduct_type = (Select product_type from products where id = NEW.product_id);
IF (vproduct_type = 0) THEN
UPDATE products set stock_weight = coalesce(stock_weight,0) + NEW.net_weight where id = NEW.product_id;
ELSE
UPDATE products set stock_pieces = coalesce(stock_pieces,0) + NEW.pieces where id = NEW.product_id;
END IF;
END $$
delimiter ;
insert into products values(1,1,null,null),(2,0,null,null);
insert into stocks values(1,null,10),(2,10,null);
select * from products;
+------+--------------+--------------+--------------+
| id | product_type | stock_weight | stock_pieces |
+------+--------------+--------------+--------------+
| 1 | 1 | NULL | 10 |
| 2 | 0 | 10 | NULL |
+------+--------------+--------------+--------------+
2 rows in set (0.001 sec)