Getting stock balance - mysql

Question

I have 3 tables as follows:
products:[product_id,product_name]
stock:[product_id,date,qty_received]
sales:[product_id,date,qty_sold]
I'm trying to display the balance of all products by subtracting qty_received - qty_sold.
My query is:
SELECT DISTINCT p.product_id, p.product_name,
IFNULL(sum(qty_received),0)-IFNULL(sum(qty_sold),0) as balance FROM products p
LEFT JOIN stock st ON (p.product_id=st.product_id)
LEFT JOIN sales sa ON (p.product_id=sa.product_id)
GROUP BY p.product_id
With the above query I'm not getting the correct balance.
Could you please help.
Additional Information:
Following are a simplified structure for my tables
Products table:
CREATE TABLE IF NOT EXISTS `products` (
`product_id` varchar(15) NOT NULL,
`product_name` varchar(20) NOT NULL,
UNIQUE KEY `product_id` (`product_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
-- Dumping data for table products
INSERT INTO `products` (`product_id`, `product_name`) VALUES
('111', 'Product One'),
('222', 'Product Two'),
('333', 'Product Three'),
('444', 'Product Four');
Stock table:
CREATE TABLE IF NOT EXISTS `stock` (
`stock_id` int(11) NOT NULL AUTO_INCREMENT,
`product_id` int(11) NOT NULL,
`qty_received` int(11) NOT NULL,
PRIMARY KEY (`stock_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=112 ;
-- Dumping data for table stock
INSERT INTO `stock` (`stock_id`, `product_id`, `qty_received`) VALUES
(1, 111, 5),
(2, 222, 10),
(3, 333, 4),
(4, 444, 6),
(5, 111, 2),
(6, 222, 7),
(7, 111, 3),
(8, 111, 3);
Sales Table
CREATE TABLE IF NOT EXISTS `sales` (
`sales_id` int(11) NOT NULL AUTO_INCREMENT,
`product_id` int(11) NOT NULL,
`qty_sold` int(11) NOT NULL,
PRIMARY KEY (`sales_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=7 ;
-- Dumping data for table sales
INSERT INTO `sales` (`sales_id`, `product_id`, `qty_sold`) VALUES
(1, 111, -1),
(2, 222, -3),
(3, 333, -2),
(4, 111, -3),
(5, 222, -1),
(6, 222, -4);
The Result I'm getting is:
+------------+--------------+---------+
| product_id | product_name | balance |
+------------+--------------+---------+
| 111 | Product One | 10 |
| 222 | Product Two | 35 |
| 333 | Product Three| 2 |
| 444 | Product Four | 6 |
+------------+--------------+---------+
Correct balance I'm expecting is
+------------+--------------+---------+
| product_id | product_name | balance |
+------------+--------------+---------+
| 111 | Product One | 9 |
| 222 | Product Two | 9 |
| 333 | Product Three| 2 |
| 444 | Product Four | 6 |
+------------+--------------+---------+
Finally got a working sql for this solution , but not sure if this is the best way doing this. I'm very concerned about the performance, since these tables got large amount of records.
SELECT p.product_id,p.product_name,
IFNULL(sum(st.qty_received),0)+IFNULL(sold,0) as balance FROM products p
LEFT JOIN stock st ON (p.product_id=st.product_id)
LEFT JOIN (SELECT sa.product_id,sum(sa.qty_sold) as sold FROM sales sa
GROUP BY sa.product_id) as t1 ON (t1.product_id=p.product_id)
GROUP BY product_id
Anyone can suggest a better sql (or change in schema) for better speed/performance to get stock balance?

Answer 1

First, add p.product_name to GROUP BY clause.
Second, remove DISTINCT - it's unnecessary with GROUP BY.
UPDATE
I've tried a couple of queries and found that this one looks like the simplest:
SELECT
p.product_id,
p.product_name,
COALESCE(SUM(qty), 0) as balance
FROM products p
LEFT JOIN (
SELECT product_id, qty_received AS qty FROM stock st UNION ALL
SELECT product_id, qty_sold FROM sales st
) AS T
ON p.product_id = T.product_id
GROUP BY
p.product_id,
p.product_name
Also, I think it would be a good idea to make data type of product_id in all your tables the same. It's varchar in products and int in sales/stock.
I did a Fiddle for you