Getting price difference between two dates - sql

Question

There is a table where once a day/hour lines are added that contain the product ID, price, name and time at which the line was added.
CREATE TABLE products
(
id integer GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
product_id integer NOT NULL,
title text NOT NULL,
price double precision NOT NULL,
checked_at timestamp with time zone DEFAULT now()
);
The data in the products table looks like this:
id
product_id
title
price
checked_at
1
1000
Watermelon
50
2022-07-19 10:00:00
2
2000
Apple
30
2022-07-19 10:00:00
3
3000
Pear
20
2022-07-19 10:00:00
4
1000
Watermelon
100
2022-07-20 10:00:00
5
2000
Apple
50
2022-07-20 10:00:00
6
3000
Pear
35
2022-07-20 10:00:00
7
1000
Watermelon
150
2022-07-21 10:00:00
8
2000
Apple
50
2022-07-21 10:00:00
9
3000
Pear
60
2022-07-21 10:00:00
I need to pass a date range (for example, from 2022-07-19 to 2022-07-21) and get the difference in prices of all unique products, that is, the answer should be like this:
product_id
title
price_difference
1000
Watermelon
100
2000
Apple
20
3000
Pear
40
I only figured out the very beginning, where I need to get the ID of all unique products in the table using DISTINCT. Next, I need to find the rows that are closest to the date range. And finally find the difference in the price of each product.

Answer 1

You could use an aggregation approach here:
SELECT product_id, title,
MAX(price) FILTER (WHERE checked_at::date = '2022-07-21') -
MAX(price) FILTER (WHERE checked_at::date = '2022-07-19') AS price_difference
FROM products
GROUP BY product_id, title
ORDER BY product_id;