Forum Discussion
Lost Customers - Issues with DAX Calculation
Good point - you have to remove some filter from the CustomerLostDate column expression.
Try this one replacing the ADDCOLUMNS in original formula:
ADDCOLUMNS ( CALCULATETABLE ( VALUES ( Customer[CustomerCode] ), Sales ), "CustomerLostDate", CALCULATE ( MAX ( Sales[Invoice Date] ), ALLEXCEPT ( Customer, Customer[CustomerCode] ) ) + [Lost Days Limit] )
Slowly changing dimensions (SCDs) are a common data warehousing practice used to capture history of what an entity looked like at a particular time. http://www.kimballgroup.com/2008/09/slowly-changing-dimensions-part-2/
I cannot connect to just the customer number (the natural/business key) because there are multiple rows in my customer table that have this key. The customer table is tied to the sales fact table via the surrogate key (an autoincremented number that provides no real value other than to uniquely identify a row and serve as a primary key for the dimension table). So I cannot simply count customer keys because that would result in incorrect numbers. To count the number of lost/recovered/new/any customers, I must do a distinct count on the customer number, which would normally be the same as the customer key if I weren't keeping history (and is the same in the DAX pattern).
So basically, I'm trying to figure out how to alter the pattern to work with a slowly changing dimension.
Yes I know what a SCO is and I know why they exist. My point is that I think this is causing the issue. I believe the lost customer formula expects the dim table to have 1 record per customer and not multiple (best guess). marcorusso would have to tell you for sure.
I am not saying it is right or wrong, but when I create models, I always take the last view of the customer as my dim table. If I need a historical view of the customer then I assume you can just load a fact table with the historical records for those customers that have changed and use DAX to extract the historical data.
Once again, this is just an assumption and hopefully Marco can give the definitive answer.
If you have an SCD2, instead of
VALUES ( Sales[CustomerNO] )
you should use
CALCULATETABLE ( VALUES ( Customer[CustomerCode] ), Sales )
where CustomerCode is the key for the customer that is the same across all the versions (surrogate keys) of the customer itself.
Please note this will slow down the performance and it is the reason why we used the column in Sales.
A best practice is to denormalize the customer code in the fact table, just to use that in this calculation.
Please note that if you are using Excel 2016 or Power BI Desktop, it is possible to write the same pattern using the new set functions (INTERSECT, EXCEPT, UNION) in a much faster way. We'll update the patterns for these versions, but I'm worried we'll not have time until this autumn.
Thank you, Marco. That gives me the correct answer if I only have the Customer No in a pivot table. If I include the Customer Name in the pivot table (this is the attribute that changed over time), it counts them as lost. Is there a way to alter the calculation to change this behavior so I get consistent results in both pivots?
For example, I have a customer who bought something in December and then their name changed, then they had purchases in January and February.
Customer CustomerKey Customer No Customer Name EffectiveStart EffectiveEnd RowIsCurrent 1 1 Person A 1/1/1900 1 2/31/2015 0 2 1 Person A2 1/1/2016 1
SalesInvoice InvoiceDateKey InvoiceDate CustomerKey ProductKey SalesAmount 20151218 12/18/2015 1 3 10 20160129 1/29/2016 2 4 15 20160216 2/12/2016 2 5 20
Their pivot with just Customer No looks good (they are not counted as lost). But if I include Customer Name after the Customer Number, they are counted as lost in February, despite the fact that I am not using the CustomerKey (surrogate key) anywhere in the formula.
Good point - you have to remove some filter from the CustomerLostDate column expression.
Try this one replacing the ADDCOLUMNS in original formula:
ADDCOLUMNS ( CALCULATETABLE ( VALUES ( Customer[CustomerCode] ), Sales ), "CustomerLostDate", CALCULATE ( MAX ( Sales[Invoice Date] ), ALLEXCEPT ( Customer, Customer[CustomerCode] ) ) + [Lost Days Limit] )