Friday, April 26, 2019

Power BI DAX #2: Meet the most popular DAX formula -- CALCULATE

Anyone wanting to do any serious data analysis with Power BI must be very good friends with CALCULATE.  Most books and online resources will tell you that CALCULATE gives you power over the filter context, enabling you to modify it as you wish. Example of one such online resource is Microsoft Documentation: 

I, however, have an easier to digest way of explaining it to people entirely new to Power BI and don't know what context means.

I assume you are already very familiar with Microsoft Excel. 

In Microsoft Excel, when you need to do a formula, you simply go into one cell and type out formulas that typically give you back one result which will show in that cell.

That is the usual usage pattern. However, there is another way of inputting formulas in Excel. If you convert the Excel data into table (Format as Table), any formula you type in the table works across the table entire rows. You don't have to drag down the formula.

How does all these relate to Power BI?

Well, the more intuitive way of entering formulas in Power BI is the second way I mentioned. Any formula you enter gets computed for the entire rows in the table. It's called New Column.

But what if you want to calculate the total sales for Enugu branch, like I did in the first Excel screenshot? Well, one has to do a New Measure, as that is what gives you one value rather than values as many as the number of rows in our table. In Excel, this would be easily solved with a SUMIF.

In Power BI, there is no SUMIF nor COUNTIF nor AVERAGEIF nor SUBTOTAL. Rather what you have is this amazing formula called CALCULATE. It makes it possible to extract out the rows/records that meet conditions you specify and then you run any normal aggregation/expression on the extract (like SUM, AVERAGE etc). So it is your SUMIF, COUNTIF, AVERAGEIF and many more common formulas you use in Excel. In fact, it is more than all of them. It does near magical things.

So how does CALCULATE work?

You specify the aggregation/expression you want and then put in filter conditions to narrow down to just the very records you want to run that expression on. 

In this very example I have shown via screenshots, here is the CALCULATE formula I entered:

Enugu Sales = CALCULATE(SUM('Pizza Sales Data'[Amount Sold]),'Pizza Sales Data'[Branch]="Enugu")

And that's how CALCULATE works.

However, don't be deceived by the simple straightforward example I have used. As you progress in your Power BI journey you will definitely have to get comfortable mixing CALCULATE with FILTER, ALL, ALLEXCEPT and many other DAX formulas. That's where the true mastery and magic lie. Also, you should read up on row context and filter context in Power BI.

You can watch the YouTube tutorial:


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1 comment:

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