Sum where exists in another table

Hi MCornish ,

Sorry for mu late respond, we can create a measure as below.

Measure = 
VAR linkcolumn =
    VALUES ( 'Table (2)'[LinkColumn] )
VAR ordernum =
    VALUES ( 'Table (2)'[OrderNum] )
RETURN
    IF (
        ISFILTERED ( 'Table (2)'[PromoID] ),
        CALCULATE (
            SUM ( 'Table'[LineCost] ),
            FILTER (
                'Table',
                'Table'[LinkColumn] IN linkcolumn
                    && 'Table'[OrderNum] IN ordernum
            )
        ),
        BLANK ()
    )

Pbix as attached as well.

v-frfei-msft 

With a small tweak it does what I need.

Measure = 
VAR linkcolumn =
    VALUES ( 'Table (2)'[LinkColumn] )
VAR ordernum =
    VALUES ( 'Table (2)'[OrderNum] )
RETURN
    IF (
        ISFILTERED ( 'Table (2)'[PromoID] ),
        CALCULATE (
            SUM ( 'Table'[LineCost] ),
            FILTER (
                'Table',
                'Table'[OrderNum] IN ordernum
            )
        ),
        BLANK ()
    )

Basically I removed the 

'Table'[LinkColumn] IN linkcolumn

from the SUM.

Cheers