Difference current month / month a year back on customer level in Power BI

Question

I am trying to calculate the sales volume difference between a given month and the same month one year back (e.g Jan 2021 - Jan 2020). The tricky part is that it should be done on customer level. My table now looks like this:

Date	Customer_number	Sales_Volume
14.01.2020	1111	1000
13.01.2020	2222	2000
21.02.2020	1111	4000
17.02.2020	2222	5000
22.01.2021	1111	1500
24.01.2021	2222	2600
19.02.2021	1111	4700
18.02.2021	2222	5800

And my output should look like this:

Date	Customer_number	Sales_Volume	Volume_Difference
14.01.2020	1111	1000	0
13.01.2020	2222	2000	0
21.02.2020	1111	4000	0
17.02.2020	2222	5000	0
22.01.2021	1111	1500	500
24.01.2021	2222	2600	600
19.02.2021	1111	4700	700
18.02.2021	2222	5800	800

Answer 1

This is for a calculated column:

Column = 
VAR _y = YEAR([Date])
VAR _m = MONTH([Date])
VAR _c = [Customer_number]
VAR _tbl_prev = FILTER(ALL('Table'), [Customer_number] = _c && MONTH([Date]) = _m && YEAR([Date]) = _y - 1)
VAR _tbl_curr = FILTER(ALL('Table'), [Customer_number] = _c && MONTH([Date]) = _m && YEAR([Date]) = _y)
VAR _result = SUMX(_tbl_curr, [Sales_Volume]) - SUMX(_tbl_prev, [Sales_Volume])
RETURN IF(COUNTROWS(_tbl_prev), _result, 0)