SUM Column but retrieve a value from another table if (blank) value

Hi owallis ,

How about create a "Value 2" column in "Forecast" table?

Value 2 = 
VAR PlanValue =
    LOOKUPVALUE (
        Plan[Value],
        Plan[Date], Forecast[Date],
        Plan[Prefix], Forecast[Prefix]
    )
RETURN
    IF ( ISBLANK ( Forecast[Value] ), PlanValue, Forecast[Value] )

Or, modified mahoneypat 's measure like so:

NewMeasure =
VAR __planvalue =
    SUM ( Plan[Value] )
VAR __forecastvalue =
    CALCULATE (
        SUM ( Forecast[Value] ),
        TREATAS ( VALUES ( Plan[Date] ), Forecast[Date] ),
        TREATAS ( VALUES ( Plan[Prefix] ), Forecast[Prefix] )
    )
RETURN
    IF ( ISBLANK ( __forecastvalue ), __planvalue, __forecastvalue )

Measure 2 =
IF (
    HASONEVALUE ( 'Date'[Date] ),
    [NewMeasure],
    SUMX ( 'Forecast', [NewMeasure] )
)

BTW, .pbix file attached.

Best Regards,

Icey

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