PowerPivot DAX Session Notes 2 – Previous Row

Those that are familiar with PowerPivot and DAX, will be aware that DAX provides the ability to reference columns.

However, it does not provide support for Cell based references (such as A1, E23).

 

This is normally no big issue, until I hit a small design issue this week… I had the requirement to refer to a previous row of data within the PowerPivot model.

And yes, it is possible, using the EARLIER DAX function.

 

Let me talk through the data quickly… I received some stock futures trading data, where each trade had two rows. One row would be the opening position at a point in time, and when the future actually closed, a second row would come through with the closing position. The profit made would be the difference between the opening position and the closing position.

 

Something like the following table…

 

 

The basic process to solve this problem was to do the following:

1. for every row, determine the Previous Date that applies to this particular Client Code.

2. Once the Date is established, determine the Value at that point in time.

3. Profit is then simply Current Value – Previous Value for CLOSE rows.

 

1. Using the following DAX statement, I could calculate the [DatePrevious] column

=CALCULATE(MAX(Table1[Date]), (FILTER(Table1, EARLIER(Table1[Code])=Table1[Code] && EARLIER(Table1[Date])>Table1[Date])))

Note: I’m still coming to grips with the EARLIER function, seems to work semantically the same as an INNER join… will hopefully post a better explanation when I understand some more…

2. Using similar thinking, I could then join back to [Table1] and return the Value at the point in time established in the previous step.

(I would love any feedback if it’s possible to merge this with the previous step, may be more efficient)

=CALCULATE(SUM(Table1[Value]), (FILTER(Table1, EARLIER(Table1[Code])=Table1[Code] && EARLIER(Table1[DatePrevious])=Table1[Date])))

3. Finally, Profit is then just the difference between the current value and the previous value for the CLOSE rows.

=if(Table1[Position]="CLOSE", Table1[Value]-Table1[ValuePrevious], BLANK())

Results in the table below.

 

PowerPivot DAX Session Notes 1 – Dates

Part of my preperation for TechEd Africa this year has required me to really get into PowerPivot and DAX, and I thought I’d share some of the lessons learnt along the way…

 

Dates

 

So, in standard Analysis Services models, the concept of a Date dimension is quite accepted… and the world of PowerPivot is not much different.

Part of creating this type of Dimension / Lookup table involves adding attributes like ‘Month’, ‘Year’, ‘Day of Week’ to a given date. Given the date expressions in Excel, it is quite straight forward to add these attributes using these standard Excel functions.

 

One that I didn’t find out the box though was a MonthName(<date>) function.

So, approaches to getting this right are:

1. Using a massive switch / case statement; yuck 🙂

2. Using a lookup table of sorts to map MonthNumber to MonthName; less yuck.

3. Use Excel date formatting features to achieve the same result… (and yes, I only learnt about this this morning, always more to learn)

 – =TEXT([@Date],”mmmm”)

 

Another common hinderance is the sort order of months.

By default sorting is alphabetical, meaning April suddenly appears before February. (and oddly enough this does confuse end users!)

 

 

Again, the more SQL based approaches would involve injecting some kind of number into the month name and sorting accordingly. (But 02-February is also not so user friendly on reports!)

Fortunately, Standard Excel sorting functions understand Months. So, if you sort the months A-Z, Excel will sort correctly.