DAX problem, filter/relation is ignored when I use IF in the RETURN clause - powerbi

Question

I'm getting an unexpected result when I use an IF in the RETURN clause of a DAX expression. If I don't use the IF, but instead just a variable, then the result is ok.
I've created a test scenario to explain my problem:
I have two test tables:
Table: "Test Object"
Table: "Test Group"
These have a unidirectional relation on "Group code"
I have created a measure "Test measure":
This gives the correct result:
I have set a page filter to only show Group Code "G01".
This all works ok up to this point.
But it goes wrong when I use an IF function:
I then get the following (incorrect) result. Apparently the relation and/or page filter seems to be ignored now:
NB: The result is the same regardless of from which table I use the "Group code" field.
What am I missing here?
I've created a PBIX file that shows the problem:
https://www.dropbox.com/s/76ld1kv503ul6nm/DAX%20problem%20with%20IF.pbix?dl=0

Answer 1

This is called "Auto-Exist" in PBI:
https://www.sqlbi.com/articles/understanding-dax-auto-exist/
If you look closer to the results, you'll notice that your report shows all possible combinations between Group Codes and Object Codes.
This is happening whenever you use a combination of fields from the different tables in a report: PBI first creates a cross-join between these fields, and then eliminates those combinations that result in blanks, so you only see meaningful combinations.
However, you IF statement overrides this logic - you are returning a result always, even if a combination is blank (Blank < 40 test returns "low end" because blank is treated as zero).
To fix it, calculate results only if the variable is not blank, i.e:
Price category =
var lowestPrice = MIN(Object[Price])
var result = IF( NOT ISBLANK(lowestPrice), IF(lowestPrice < 40, "Low end", "High end"))
Return result
You will get:
P.S. Page filter is irrelevant here, it simply filters the table after it's calculated.