Rolling average in Power Query

[cho]

Hi Experts!

I have a table (after few steps to the original table) with below columns (sample):

Currency	YearMonth	Sales
USD	201912	1.1234
CZK	201912	25.408
GBP	201912	0.8508
RON	201912	4.783
CNY	201912	7.8205
EUR	201912	1
USD	202001	1.1052
CZK	202001	25.21
GBP	202001	0.84175
RON	202001	4.7789
CNY	202001	7.6664
EUR	202001	1
USD	202002	1.0977
CZK	202002	25.39
GBP	202002	0.85315
RON	202002	4.813
CNY	202002	7.6662
EUR	202002	1
USD	202003	1.0956
CZK	202003	27.312
GBP	202003	0.88643
RON	202003	4.8283
CNY	202003	7.7784
EUR	202003	1

I need a custom column with below condition:
From YearMonth column, pick the last months' Sale, starting from January of that particular year to that particular month, including last year's December value, and take the average of those Sales according to that Currency.

I added a custom column with a code like this:

let
    currency = [Currency],
    year = Number.FromText(Text.Start(Text.From([YearMonth]), 4)),
    month = Number.FromText(Text.End(Text.From([YearMonth]), 2)),
    months_to_include = List.Numbers(
        if month < 7 then 12 - month + 1 else 1,
        if month < 7 then 6 else month,
        1
    ),
    last_year_rows = Table.SelectRows(
        #"Removed Other Columns",
        each [Currency] = currency and
             Number.FromText(Text.Start(Text.From([YearMonth]), 4)) = year - 1 and
             Number.FromText(Text.End(Text.From([YearMonth]), 2)) > 6 - month and
             Number.FromText(Text.End(Text.From([YearMonth]), 2)) <= 12
    ),
    current_year_rows = Table.SelectRows(
        #"Removed Other Columns",
        each [CURRENCY_ID] = currency and
             Number.FromText(Text.Start(Text.From([YearMonth]), 4)) = year and
             Number.FromText(Text.End(Text.From([YearMonth]), 2)) <= month
    ),
    all_rows = current_year_rows & last_year_rows,
    filtered_rows = Table.SelectRows(
        all_rows,
        each [YearMonth] >= Number.ToText(year * 100 + months_to_include{0}, "0000") and [YearMonth] <= [YearMonth]
    ),
    avg_Sales = List.Average(filtered_rows[Sales])
in
    avg_Sales

But now, I although values are correct after July, from January to June values are NULL. Can someone please help me to fix this?
I understand the error should be in "months_to_include", but I do not know how to fix it. Thank you!

[ppm1 · Accepted solution]

This type of calculation is much easier with DAX (and a Date column vs. date integer). Have you already tried that?

Pat

[cho]

Yes, I added a calculated column like this but it gives wrong values. 

Average= 
VAR currentYearMonth = [YearMonth]
VAR currentMonth = RIGHT(currentYearMonth, 2)
VAR currentYear = LEFT(currentYearMonth, 4)
VAR lastYearMonth = IF(currentMonth = "01", FORMAT(VALUE(currentYear) - 1, "0000") & "12", FORMAT(VALUE(currentYear), "0000") & FORMAT(VALUE(currentMonth) - 1, "00"))
VAR numMonths = VALUE(currentMonth) + (12 * (VALUE(currentYear) - VALUE(LEFT(lastYearMonth, 4))))
RETURN
CALCULATE(
    AVERAGE('table'[Sales]),
    FILTER(
        'table',
        'table'[Currency] = EARLIER('table'[currency]) &&
        'table'[YearMonth] >= lastYearMonth &&
        'table'[YearMonth] <= currentYearMonth
    )
)

[cho]

Since there are other calculations those cannot be done in desktop, I need to do this in Power Query 😞