Assign total value of month to each day of month - sql

Question

DB-Fiddle
CREATE TABLE sales (
id SERIAL PRIMARY KEY,
country VARCHAR(255),
sales_date DATE,
sales_volume DECIMAL,
fix_costs DECIMAL
);
INSERT INTO sales
(country, sales_date, sales_volume, fix_costs
)
VALUES
('DE', '2020-01-03', '500', '0'),
('FR', '2020-01-03', '350', '0'),
('None', '2020-01-31', '0', '2000'),
('DE', '2020-02-15', '0', '0'),
('FR', '2020-02-15', '0', '0'),
('None', '2020-02-29', '0', '5000'),
('DE', '2020-03-27', '180', '0'),
('FR', '2020-03-27', '970', '0'),
('None', '2020-03-31', '0', '4000');
Expected Result:
sales_date | country | sales_volume | fix_costs
--------------|-------------|-------------------|-----------------
2020-01-03 | DE | 500 | 2000
2020-01-03 | FR | 350 | 2000
2020-02-15 | DE | 0 | 5000
2020-02-15 | FR | 0 | 5000
2020-03-27 | DE | 180 | 4000
2020-03-27 | FR | 970 | 4000
As you can see in my table I have a total of fix_costs assigned to the last day of each month.
In my results I want to assign this total of fix_costs to each day of the month.
Therefore, I tried to go with this query:
SELECT
s.sales_date,
s.country,
s.sales_volume,
f.fix_costs
FROM sales s
JOIN
(SELECT
((date_trunc('MONTH', sales_date) + INTERVAL '1 MONTH - 1 DAY')::date) AS month_ld,
SUM(fix_costs) AS fix_costs
FROM sales
WHERE country = 'None'
GROUP BY month_ld) f ON f.month_ld = LAST_DAY(s.sales_date)
WHERE country <> 'None'
GROUP BY 1,2,3;
For this query I get an error on the LAST_DAY(s.sales_date) since this expression does not exist in PostgresSQL.
However, I have no clue how I can replace it correctly in order to get the expected result.
Can you help me?
(MariaDB Fiddle as comparison)

Answer 1

demos:db<>fiddle
SELECT
s1.sales_date,
s1.country,
s1.sales_volume,
s2.fix_costs
FROM sales s1
JOIN sales s2 ON s1.country <> 'None' AND s2.country = 'None'
AND date_trunc('month', s1.sales_date) = date_trunc('month', s2.sales_date)
You need a natural self-join. Join conditions are:
First table without None records (s1.country <> 'None')
Second table only None records (s2.country = 'None')
Date: Only consider year and month part, ignore days. This can be achieved by normalizing the dates of both tables to the first of the month by using date_trunc(). So, e.g. '2020-02-15' results in '2020-02-01' and '2020-02-29' results in '2020-02-01' too, which works well as comparision and join condition.
Alternatively:
SELECT
*
FROM (
SELECT
sales_date,
country,
sales_volume,
SUM(fix_costs) OVER (PARTITION BY date_trunc('month', sales_date)) as fix_costs
FROM sales
) s
WHERE country <> 'None'
You can use the SUM() window function over the group of date_trunc() as described above. Then you need filter the None records afterwards

Answer 2

If I understand correctly, use window functions:
select s.*,
sum(fix_costs) over (partition by date_trunc(sales_date)) as month_fixed_costs
from sales;
Note that this assumes that fixed costs are NULL or 0 on other days -- which is true for the data in the question.