Subtract Values per week

Hi Syndicate_Admin 

At first, try my code to build a date table. If you use weeknum, you will get confused at the begining of next year.

For example, 2020/12/31 and 2021/01/01 are in the same week, but weeknum will show you 53 and 1. That's incorrect and will make our calculate difficult. 

Date table:

Date =
ADDCOLUMNS (
    CALENDARAUTO (),
    "Year", YEAR ( [Date] ),
    "Month", MONTH ( [Date] ),
    "Weeknum", WEEKNUM ( [Date], 2 ),
    "YearMonth",
        YEAR ( [Date] ) * 100
            + MONTH ( [Date] )
)

Add calculated columns:

ISO 8601 WeekNum = 
VAR _COUNT0 =
    CALCULATE (
        COUNTROWS ( 'Date' ),
        FILTER (
            'Date',
            'Date'[Year] = EARLIER ( 'Date'[Year] )
                && 'Date'[WeekNum] - 1 = 0
        )
    )
VAR _BASENUM1 =
    IF ( _COUNT0 < 7, 'Date'[WeekNum] - 1, 'Date'[WeekNum] )
VAR _ISO_8601_WeekNum =
    IF (
        WEEKDAY ( DATE ( 'Date'[Year] - 1, 01, 01 ) ) <> 1
            && 'Date'[Year] = 'Date'[Year]
            && _BASENUM1 = 0,
        WEEKNUM ( DATE ( MIN ( 'Date'[Year] ), 12, 31 ), 1 ) - 1,
        _BASENUM1
    )
RETURN
    _ISO_8601_WeekNum

ISO_Year = 
VAR _COUNT0 =
    CALCULATE (
        COUNTROWS ( 'Date' ),
        FILTER (
            'Date',
            'Date'[Year] = EARLIER ( 'Date'[Year] )
                && 'Date'[WeekNum] - 1 = 0
        )
    )
VAR _BASENUM1 =
    IF ( _COUNT0 < 7, 'Date'[WeekNum] - 1, 'Date'[WeekNum] )
RETURN
IF(_BASENUM1 = 0,'Date'[Year] -1,'Date'[Year])

ISO YearWeekNum = 'Date'[ISO_Year]*100+'Date'[ISO 8601 WeekNum]

Build a relationship between your data table and this calendar date table by date column.

Create a measure as below.

Diff =
VAR _CurValue =
    CALCULATE ( SUM ( 'Sample'[Value] ) )
VAR _LastISOYearWeekNum =
    MAXX (
        FILTER (
            ALL ( 'Date' ),
            'Date'[ISO YearWeekNum] < MAX ( 'Date'[ISO YearWeekNum] )
        ),
        'Date'[ISO YearWeekNum]
    )
VAR _LastValue =
    CALCULATE (
        SUM ( 'Sample'[Value] ),
        FILTER ( ALL ( 'Date' ), 'Date'[ISO YearWeekNum] = _LastISOYearWeekNum )
    )
RETURN
    _CurValue - _LastValue

Result is as below.

Best Regards,
Rico Zhou

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