Summing up a related table's values in PowerPivot/DAX

问题

Say I have two tables. attrsTable:

file | attribute | value
------------------------
A    | xdim      | 5
A    | ydim      | 6
B    | xdim      | 7
B    | ydim      | 3
B    | zdim      | 2
C    | xdim      | 1
C    | ydim      | 7

sizeTable:

file | size
-----------
A    | 17
B    | 23
C    | 34

I have these tables related via the 'file' field. I want a PowerPivot measure within attrsTable, whose calculation uses size. For example, let's say I want xdim+ydim/size for each of A, B, C. The calculations would be:

A: (5+6)/17
B: (7+3)/23
C: (1+7)/34

I want the measure to be generic enough so I can use slicers later on to slice by file or attribute. How do I accomplish this?

I tried:

dimPerSize := CALCULATE([value]/SUM(sizeTable[size])) # Calculates 0
dimPerSize := CALCULATE([value]/SUM(RELATED(sizeTable[size]))) # Produces an error

Any idea what I'm doing wrong? I'm probably missing some fundamental concepts here of how to use DAX with relationships.

回答1:

Hi Redstreet,

taking a step back from your solution and the one proposed by Jacob, I think it might be useful to create another table that would aggregate all the calculations (especially given you probably have more than 2 tables with file-specific attributes).

So I have created one more table that contains (only) unique file names, and thus the relationships could be visualized this way:

It's much simpler to add necessary measures (no need for calculated columns). I have actually tested 2 scenarios:

1) create simple SUM measures for both Attribute Value and File Size. Then divide those two measures and job done :-).

2) use SUMX functions to have a bit more universal solution. Then the final formula for DimPerSize calculation could look like this:

=DIVIDE(
   SUMX(DISTINCT(fileTable[file]),[Sum of AttrValue]), 
   SUMX(DISTINCT(fileTable[file]),[Sum of FileSize]), 
   BLANK()
)

With [Sum of AttrValue] being:

=SUM(attrsTable[value])

And Sum of FileSize being:

=SUM(sizeTable[size])

This worked perfectly fine, even though SUMX in both cases goes over all instances of given file name. So for file B it also calculates with zdim (if there is a need to filter this out, then use simple calculate / filter combination). In case of file size, I am using SUMX as well, even though it's not really needed since the table contains only 1 record for each file name. If there would be 2 instances, then use SUMX or AVERAGEX depending on the desired outcome.

This is the link to my source file in Excel (2010).

Hope this helps.

回答2:

You look to have the concept of relationships OK but you aren't on the right track in terms of CALCULATE() either in terms of the structure or the fact that you can't simply use 'naked' numerical columns, they need to be packaged in some way.

Your desired approach is correct in that once you get a simple version of the thing running, you will be able to slice and dice it over any of your related dimensions.

Best practice is probably to build this up using several measures:

[xdim] = CALCULATE(SUM('attrstable'[value]), 'attrstable'[attribute] = "xdim")
[ydim] = CALCULATE(SUM('attrstable'[value]), 'attrstable'[attribute] = "ydim")
[dimPerSize] = ([xdim] + [ydim]) / VALUES('sizeTable'[size])

But depending on exactly how your pivot is set up, this is likely to also throw an error because it will try and use the whole 'size' column in your totals. There are two main strategies for dealing with this:

Use an 'iterative' formula such as SUX() or AVERAGEX() to iterate individually over the 'file' field and then adds up or averages for the total e.g.

[ItdimPerSize] = AVERAGEX(VALUES('sizeTable'[file]), [dimPerSize])

Depending on the maths you want to use, you might find that produce a useful average that you need to use SUMX but devide by the number of cases i.e. COUNTROWS('sizeTable'[file]).
You might decide that the totals are irrelevant and simply introduce an error handling element that will make them blank e.g.

[NtdimPerSize] = IF(HASONEVALUE('sizeTable'[file]),[dimPerSize],BLANK())

NB, all of this assumes that when you are creating your pivot that you are 'dragging in' the file field from the 'sizetable'.

来源：https://stackoverflow.com/questions/25345181/summing-up-a-related-tables-values-in-powerpivot-dax

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