MariaDB 10.3

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE sales (
    id int auto_increment primary key,
    country VARCHAR(255),
    sales_date DATE,
    sales_volume INT,
    fix_costs INT
);

INSERT INTO sales
(country, sales_date, sales_volume, fix_costs
)
VALUES 

("DE", "2020-01-03", "500", "0"),
("NL", "2020-01-03", "320", "0"),
("FR", "2020-01-03", "350", "0"),
("None", "2020-01-30", "0", "2000"),

("DE", "2020-02-15", "700", "0"),
("NL", "2020-02-15", "420", "0"),
("FR", "2020-02-15", "180", "0"),
("None", "2020-02-29", "0", "5000"),

("DE", "2020-03-27", "180", "0"),
("NL", "2020-03-27", "670", "0"),
("FR", "2020-03-27", "970", "0"),
("None", "2020-03-31", "0", "4000");
select 
	sales_date, 
	country, 
	sum(sales_volume),
	sum(sum(fix_costs)) over(partition by year(sales_date), month(sales_date))
		/ day(last_day(sales_date)) 
		* sum(sales_volume)
		/ sum(sum(sales_volume)) over(partition by sales_date)
		as fix_cost_per_day
from sales
group by 1,2;


sales_date
country
sum(sales_volume)
fix_cost_per_day


2020-01-03
DE
500
27.57099531

2020-01-03
FR
350
19.29969672

2020-01-03
NL
320
17.64543700

2020-01-30
None
0
null

2020-02-15
DE
700
92.83819629

2020-02-15
FR
180
23.87267905

2020-02-15
NL
420
55.70291777

2020-02-29
None
0
null

2020-03-27
DE
180
12.76143212

2020-03-27
FR
970
68.76993974

2020-03-27
NL
670
47.50088621

2020-03-31
None
0
null


select 
    sales_date, 
    country, 
    sum(sales_volume),
    case when country <> 'NL'
		then sum(sum(fix_costs)) over(partition by year(sales_date), month(sales_date))
			/ day(last_day(sales_date)) 
			* sum(sales_volume)
			/ sum(case when country <> 'NL' then sum(sales_volume) else 0 end) over(partition by sales_date)
	end as fix_cost_per_day
from sales
group by 1,2;


sales_date
country
sum(sales_volume)
fix_cost_per_day


2020-01-03
DE
500
37.95066414

2020-01-03
FR
350
26.56546490

2020-01-03
NL
320
null

2020-01-30
None
0
null

2020-02-15
DE
700
137.14733542

2020-02-15
FR
180
35.26645768

2020-02-15
NL
420
null

2020-02-29
None
0
null

2020-03-27
DE
180
20.19635344

2020-03-27
FR
970
108.83590463

2020-03-27
NL
670
null

2020-03-31
None
0
null

sales_date	country	sum(sales_volume)	fix_cost_per_day
2020-01-03	DE	500	27.57099531
2020-01-03	FR	350	19.29969672
2020-01-03	NL	320	17.64543700
2020-01-30	None	0	null
2020-02-15	DE	700	92.83819629
2020-02-15	FR	180	23.87267905
2020-02-15	NL	420	55.70291777
2020-02-29	None	0	null
2020-03-27	DE	180	12.76143212
2020-03-27	FR	970	68.76993974
2020-03-27	NL	670	47.50088621
2020-03-31	None	0	null

sales_date	country	sum(sales_volume)	fix_cost_per_day
2020-01-03	DE	500	37.95066414
2020-01-03	FR	350	26.56546490
2020-01-03	NL	320	null
2020-01-30	None	0	null
2020-02-15	DE	700	137.14733542
2020-02-15	FR	180	35.26645768
2020-02-15	NL	420	null
2020-02-29	None	0	null
2020-03-27	DE	180	20.19635344
2020-03-27	FR	970	108.83590463
2020-03-27	NL	670	null
2020-03-31	None	0	null