Split fix value to countries based on pre-defined share per country - sql

Question

DB-Fiddle
CREATE TABLE costs (
id SERIAL PRIMARY KEY,
event_date DATE,
country VARCHAR,
channel VARCHAR,
costs DECIMAL
);
INSERT INTO costs
(event_date, country, channel, costs)
VALUES
('2020-02-08', 'DE', 'channel_01', '400'),
('2020-02-08', 'DE', 'channel_02', '400'),
('2020-02-08', 'DE', 'channel_03', '400'),
('2020-02-08', 'FR', 'channel_01', '400'),
('2020-02-08', 'FR', 'channel_02', '400'),
('2020-02-08', 'NL', 'channel_01', '400'),
('2020-04-15', 'DE', 'channel_01', '300'),
('2020-04-15', 'FR', 'channel_01', '300'),
('2020-04-15', 'NL', 'channel_01', '300'),
('2020-04-15', 'NL', 'channel_02', '300'),
('2020-04-15', 'NL', 'channel_03', '300');
Expected Result:
event_date | country | costs |
--------------|--------------|-----------------------------|---------
2020-02-08 | DE | 240 (=400 x 0.6) |
2020-02-08 | FR | 120 (=400 x 0.3) |
2020-02-08 | NL | 40 (=400 x 0.1) |
--------------|--------------|-----------------------------|---------
2020-04-15 | DE | 180 (=300 x 0.6) |
2020-04-15 | FR | 90 (=300 x 0.3) |
2020-04-15 | NL | 30 (=300 x 0.1) |
I want to split the costs based on pre-defined shares to each country per day.
The shares are: DE = 0.6, FR = 0.3, NL = 0.1
SELECT
c.event_date,
c.country,
c.costs
FROM costs c
GROUP BY 1,2,3
ORDER BY 1,2;
Do you have an idea what query I need to achieve this?

Answer 1

Just use a CASE expression:
SELECT c.event_date,
c.country,
(CASE WHEN c.country = 'DE' THEN 0.6 * c.costs
WHEN c.country = 'FR' THEN 0.3 * c.costs
WHEN c.country = 'NL' THEN 0.1 * c.costs
END) as allocated_costs
FROM costs c
GROUP BY c.event_date, c.country, c.costs
ORDER BY 1, 2;
You can more conveniently store the values in a derived table, if you prefer:
SELECT c.event_date, c.country, (v.alloc * c.costs) as allocated_costs
FROM costs c JOIN
(VALUES ('DE', 0.6), ('FR', 0.3), ('NL', 0.1)
) v(country, alloc)
USING (country)
GROUP BY c.event_date, c.country, c.costs, v.alloc
ORDER BY 1, 2;
Here is a db<>fiddle.

Answer 2

the best way would be to add those rates into a table and use that table in your query.
to illustrate how you can do it with hard-coded values in your query:
SELECT
c.event_date,
c.country,
c.costs * case country when 'DE' then 0.6
when 'FR' then 0.3
when 'NL' then 0.1
end
FROM costs c
GROUP BY 1,2,3
ORDER BY 1,2;