I found IsNull() to be flaky and return unexpected results sometimes. Instead, I'm using the following formula that also covers an empty string or any number of spaces instead of the value:
if(len(trim("Risk factor"))) = 0, 0 ,"Risk factor") as RiskFactor
Another possible way to convert nulls and zeros into zeros is to use the RangeSum() function :
RangeSum("Risk factor") as RiskFactor
Any nulls will get converted into zeros.
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