DAX conditional sum

Question

How to construct a DAX measure which returns sum of either A or B. The logic is take B if A is empty. So expected results looks like this:

+---+---+----------+
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Answer 1

I'd recommend using a SUMX iterator in this case.

Measure = SUMX ( tab, IF ( ISBLANK ( tab[A] ), tab[B], tab[A] ) )

You might be able to do the following as well:

Measure = 
    CALCULATE ( SUM ( tab[A] ) ) +
    CALCULATE ( SUM ( tab[B] ),
        FILTER ( tab, ISBLANK( tab[A] ) )
    )