Postgresql, Get the top 5 products that have increased in value from yesterday to today, returning the delta - postgresql

Question

I have a pricing table that contains the pricing data for products. There are around 600 unique product_id, each currently having 4 days worth of pricing data, which will eventually go up to 30 days. The table below is a small subset of the data to represent that table structure:
date
product_id
price_trend
2022-08-21
1
0.08
2022-08-22
1
0.18
2022-08-23
1
0.30
2022-08-21
2
0.15
2022-08-22
2
0.20
2022-08-23
2
0.22
So in my script, for each product_id I am trying to get yesterdays price_trend and todays price_trend and then calculate the price_delta between the two. I then order by price_delta and limit the results to 5.
I am having some issues as in some cases yesterdays price_trend is 0 and then todays price trend is 0.50 for example. This does not mean that the price trend has increased, but mostly likely that price_trend was not gathered yesterday for whatever reason.
Now I would like to remove any records where price_trend for today or yesterday equals 0, however, when I add AND pricing.trend_price > 0 the value return is just null instead.
Script:
SELECT
magic_sets_cards.name,
(SELECT pricing.trend_price
FROM pricing
WHERE pricing.product_id = magic_sets_cards_identifiers.mcm_id
AND pricing.date = (SELECT MAX(date) - INTERVAL '2 DAY' FROM pricing)
AND pricing.trend_price > 0) AS price_yesterday,
(SELECT pricing.trend_price
FROM pricing
WHERE pricing.product_id = magic_sets_cards_identifiers.mcm_id
AND pricing.date = (SELECT MAX(date) FROM pricing)
AND pricing.trend_price > 0) AS price_today,
((SELECT pricing.trend_price
FROM pricing
WHERE pricing.product_id = magic_sets_cards_identifiers.mcm_id
AND pricing.date = (SELECT MAX(date) FROM pricing)) -
(SELECT pricing.trend_price
FROM pricing
WHERE pricing.product_id = magic_sets_cards_identifiers.mcm_id
AND pricing.date = (SELECT MAX(date) - INTERVAL '2 DAY' FROM pricing))) AS price_delta
FROM magic_sets
JOIN magic_sets_cards ON magic_sets_cards.set_id = magic_sets.id
JOIN magic_sets_cards_identifiers ON magic_sets_cards_identifiers.card_id = magic_sets_cards.id
JOIN pricing ON pricing.product_id = magic_sets_cards_identifiers.mcm_id
WHERE magic_sets.code = '2X2'
AND pricing.date = (SELECT MAX(date) FROM pricing)
ORDER BY price_delta DESC
LIMIT 5
Results:
name
price_yesterday
price_today
price_delta
"Fiery Justice"
null
0.50
0.50
"Hostage Taker"
3.50
4.00
0.50
"Damnation"
17.02
17.33
0.31
"Bring to Light"
0.42
0.72
0.30
"City of Brass"
17.41
17.68
0.27
I would like to get it so that the "Fiery Justice" in this example is just ignored.

Answer 1

with the use of rank() you can get the output ., Look into...
Query without null rows :
with cte as (Select
product_id,
SUM(CASE WHEN rank = 1 THEN price_trend ELSE null END) today,
SUM(CASE WHEN rank = 2 THEN price_trend ELSE null END) yesterday,
SUM(CASE WHEN rank = 1 THEN price_trend ELSE 0 END) -
SUM(CASE WHEN rank = 2 THEN price_trend ELSE 0 END) as diff
FROM (
SELECT
product_id,
price_trend,
date,
rank() OVER (PARTITION BY product_id ORDER BY date DESC) as rank
FROM tableName where price_trend>0 and date between current_date-5 and current_date-4) p
WHERE rank in (1,2)
GROUP BY product_id
) select * from cte where (case when today is null or yesterday is null then 'NULL' else 'VALID' end)!='NULL'
Query with null values :
Select
product_id,
SUM(CASE WHEN rank = 1 THEN price_trend ELSE 0 END) today,
SUM(CASE WHEN rank = 2 THEN price_trend ELSE 0 END) yesterday,
SUM(CASE WHEN rank = 1 THEN price_trend ELSE 0 END) -
SUM(CASE WHEN rank = 2 THEN price_trend ELSE 0 END) as diff
FROM (
SELECT
product_id,
price_trend,
date,
rank() OVER (PARTITION BY product_id ORDER BY date DESC) as rank
FROM tableName where date between current_date-5 and current_date-4) p
WHERE rank in (1,2)
GROUP BY product_id
Change the condition :
where date between current_date-3 and current_date-2
OUTPUT :
product_id today yesterday diff
1 0.06 0.02 0.04
2 0.64 0.62 0.02
CREATE TABLE tableName
(
date date,
product_id int,
price_trend numeric(9,2)
);
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-21 ', '1 ', '0.02');
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-22 ', '1 ', '0.06');
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-23 ', '1 ', '0.10');
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-24 ', '1 ', '0.13');
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-25 ', '1 ', '0.18');
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-26 ', '1 ', '0.30');
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-21 ', '2 ', '0.62');
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-22 ', '2 ', '0.64');
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-23 ', '2 ', '0.69');
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-24 ', '2 ', '0.78');
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-25 ', '2 ', '0.88');
INSERT INTO tableName (date ,product_id ,price_trend) VALUES ('2022-08-26 ', '2 ', '0.90');