Merging columns from two tables

Hi cscollier2 ,

I think you'll want to do this in DAX. Conditional merges in Power Query can be complicated and provide poor performance.

I'm normally totally against the use of DAX calculated columns, but the following is the most performant way of dealing with SCD tables I've found so far. I've got a 1M row table that I do this twice on, the second column referencing the first, and it's lightning-fast. However, I've not tested this on tables over 1M rows, so YMMV.

As a DAX calculated column on your Sales table:

..saleDiscount = 
CALCULATE(
    // Add a variable for each dimension that you need to match on
    VAR __departmentRow = VALUES(salesTable[department])
    VAR __salesDateRow = VALUES(salesTable[salesDate])
    RETURN
    MAXX(
        FILTER(
            discountSCDTable,
            // Match the different variables to the discountSCD table
            discountSCDTable[department] = __departmentRow
            && discountSCDTable[discountStartDate] <= __salesDateRow
            && discountSCDTable[discountEndDate] >= __salesDateRow
        ),
        discountSCDTable[discountRate]
    )
)

Pete