Calculated column to compare aggregated value from a different row

[gsmith_eqd]

I have aggregated the monthly sales from my salesorder table to a table containing the monthly sales information. What I'm trying to do is create a column that references the monthly sales from the previous year to show the monthly change.

This is a model of what I have so far:

Month	Year	ThisMonthSales
January	2018	$1000
January	2019	$2000
January	2020	$3000
January	2021	$4000
February	2019	$1500
February	2020	$2500

This is what I'd like to create:

Month	Year	ThisMonthSales	LastYearMonthSales
January	2018	$1000
January	2019	$2000	$1000
January	2020	$3000	$2000
January	2021	$4000	$3000
February	2019	$1500
February	2020	$2500	$1500

I tried this command, but the issue I'm running into is that the calculated column contains the yearly aggregated sales instead of the monthly sales:

Column =
VAR Last_Year = 'Table'[Year] - 1
RETURN
CALCULATE(SUM('Table'[This Month Sales]), ALL('Table'), 'Table'[Year] = Last_Year, 'Table'[Month] = 'Table'[Month])

What's the best way for me to accomplish this?

[tamerj1 · Accepted solution]

@gsmith_eqd 
please try

Column =
VAR Last_Year = 'Table'[Year] - 1
RETURN
CALCULATE (
SUM ( 'Table'[This Month Sales] ),
ALLEXCEPT ( 'Table', 'Table'[Month] ),
'Table'[Year] = Last_Year
)