Calculate balance amount per prodcuct in redshift - sql

Question

DB-Fiddle
CREATE TABLE inventory (
id SERIAL PRIMARY KEY,
stock_date DATE,
product VARCHAR(255),
inbound_quantity INT,
outbound_quantity INT
);
INSERT INTO inventory
(stock_date, product, inbound_quantity, outbound_quantity
)
VALUES
('2020-01-01', 'Product_A', '900', '0'),
('2020-01-02', 'Product_A', '0', '300'),
('2020-01-03', 'Product_A', '400', '250'),
('2020-01-04', 'Product_A', '0', '100'),
('2020-01-05', 'Product_A', '700', '500'),
('2020-01-03', 'Product_B', '850', '0'),
('2020-01-08', 'Product_B', '100', '120'),
('2020-02-20', 'Product_B', '0', '360'),
('2020-02-25', 'Product_B', '410', '230'),
Expected Result:
stock_date
product
inbound_quantity
outbound_quantity
balance
2020-01-01
Product_A
900
0
900
2020-01-02
Product_A
0
300
600
2020-01-03
Product_A
400
250
750
2020-01-04
Product_A
0
100
650
2020-01-05
Product_A
700
500
850
2020-01-03
Product_B
740
0
740
2020-01-08
Product_B
100
120
720
2020-02-20
Product_B
0
360
360
2020-02-25
Product_B
410
230
540
2020-03-09
Product_B
290
0
830
I want to calculate the balance per product.
So far I have been able to develop this query below but it does not work.
I get error window "product" does not exist.
SELECT
iv.stock_date AS stock_date,
iv.product AS product,
iv.inbound_quantity AS inbound_quantity,
iv.outbound_quantity AS outbound_quantity,
SUM(iv.inbound_quantity - iv.outbound_quantity) OVER
(product ORDER BY stock_date ASC ROWS UNBOUNDED PRECEDING) AS Balance
FROM inventory iv
GROUP BY 1,2,3,4
ORDER BY 2,1;
How do I need to modify the query to make it work?

Answer 1

You are almost there
You should add partition by in front of product
SELECT
iv.stock_date AS stock_date,
iv.product AS product,
iv.inbound_quantity AS inbound_quantity,
iv.outbound_quantity AS outbound_quantity,
SUM(iv.inbound_quantity - iv.outbound_quantity) OVER
(partition by product ORDER BY stock_date ASC ROWS UNBOUNDED PRECEDING) AS Balance
FROM inventory iv
GROUP BY 1,2,3,4
ORDER BY 2,1;
So, it should be like this

Answer 2

The error message "product" does not exist is because you are trying to reference the column "product" in the OVER clause, but it is not included in the GROUP BY clause.
To fix this issue, you will need to include the "product" column in the GROUP BY clause, and also add a partition by clause to the OVER clause, so that the SUM function will calculate the balance per product.
Try this query:
SELECT
iv.stock_date AS stock_date,
iv.product AS product,
iv.inbound_quantity AS inbound_quantity,
iv.outbound_quantity AS outbound_quantity,
SUM(iv.inbound_quantity - iv.outbound_quantity) OVER
(PARTITION BY product ORDER BY stock_date ASC ROWS UNBOUNDED PRECEDING) AS Balance
FROM inventory iv
GROUP BY 1,2,3,4
ORDER BY 2,1;
This way, SUM function will only sum the inbound_quantity - outbound_quantity for each product, and not for all the products.
The query will return the expected result.