Display only certain columns in results despite case statement in query - sql

Question

DB-Fiddle
CREATE TABLE inventory (
id SERIAL PRIMARY KEY,
product VARCHAR,
quantity DECIMAL,
avg_price DECIMAL,
normal_price DECIMAL
);
INSERT INTO inventory
(product, quantity, avg_price, normal_price)
VALUES
('product_01', '800', '10', '10'),
('product_01', '300', '20', '90'),
('product_01', '200', '0', '50'),
('product_01', '500', '30', '80'),
('product_01', '600', '0', '60'),
('product_01', '400', '50', '40');
Expected Result:
product | quantity | final_price |
-------------|--------------|----------------|--------------
product_01 | 800 | 10 |
product_02 | 300 | 20 |
product_03 | 200 | 50 |
product_04 | 500 | 30 |
product_05 | 600 | 60 |
product_06 | 400 | 50 |
I only want to display the column quantity and final_price.
However, I have to use a CASE statement in my query and the syntax from postgresSQL is forcing me to add the column avg_price and normal_price to the query in order to make the CASE statement work:
SELECT
iv.product AS product,
iv.avg_price AS avg_price,
iv.normal_price AS normal_price,
SUM(iv.quantity) AS quantity,
(CASE WHEN iv.avg_price = 0 THEN iv.normal_price ELSE iv.avg_price END) AS final_price
FROM inventory iv
GROUP BY 1,2,3
ORDER BY 1;
Not sure if this is possible in postgresSQL but is there a way to only display the two columns as in the expected result?

Answer 1

I would suggest aggregating by the expression itself:
SELECT iv.product AS product,
SUM(iv.quantity) AS quantity,
(CASE WHEN iv.avg_price = 0 THEN iv.normal_price ELSE iv.avg_price END) AS final_price
FROM inventory iv
GROUP BY iv.product, final_price
ORDER BY 1;

Answer 2

Use explicit GROUP BY iv.product, iv.avg_price, iv.normal_price instead of GROUP BY 1, 2, 3:
SELECT
iv.product AS product,
SUM(iv.quantity) AS quantity,
(CASE WHEN iv.avg_price = 0 THEN iv.normal_price ELSE iv.avg_price END) AS final_price
FROM inventory iv
GROUP BY iv.product, iv.avg_price, iv.normal_price
ORDER BY 1;