Power BI DAX Variables are an incredibly useful tool when creating long complex formulas that help you keep track of different tables and fields being referenced throughout. They have many benefits including making formulas easier to read, more modular, and help you hedge against changes that may occur within the underlying data model.
We’ll explain what variables are, why you should use them, and alternatives to consider, such as nesting measures.
Let’s jump in!
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What are Variables in Computer Programming?
The concept of variables dates back to the 1950s with the advent of computer programming languages like FORTRAN that introduced the ability to provide nicknames for storing data. By providing a block of code a nickname, it allowed programmers to quickly reference that code without having to re-write it.
This seemingly simple change saved programmers an incredible amount of time avoiding the need to re-create portions of code, and also made the code much easier to read and understand.
While Power BI DAX formulas for the most part fall short of computer programming, the concept and the benefits are similar for report builders.
What are Variables in Power BI?
Variables are used in Power BI to give nicknames to specific fields and tables that are referenced in a formula. They give Power BI developers the ability to use simple easy to understand words in place of a full table and field reference, which in some data models can be quite long or difficult to remember which field is the right one.
Similar to the use of variables in computer programming, they simplify complex expressions, improve readability, and ease of use.
Benefits of Using Variables in Power BI DAX Formulas
Leverage Variables in DAX formulas offers several benefits, some are for the Power BI developer themselves, making reports easier to manage, update and write formulas for, while others have a direct impact on the query performance and speed at which a Power BI runs.
Benefits for Power BI Developers
For developers, using variables has a couple of benefits in dashboard development and writing queries.
Readability and Maintenance
By declaring a variable, you can give a table and field name a simpler, shorter, easier to understand name that can be referenced throughout a DAX query, making it easier to read. If the table name and field names change any point, you can update the formula in a single place vs having to update it in multiple.
Consistency in Results
Variables ensure consistency in calculations. If a part of a formula needs to be calculated multiple times within a single expression, using a variable ensures that the exact same value is used each time. This safeguards you from calculation discrepancies that might arise from a typo or referencing an incorrect field.
Benefits for Speed and Query Optimization
One of the more interesting benefits of utilizing variables in DAX is the impact on query performance. For smaller datasets and simpler calculations, Power BI is already very fast. However, as datasets grow and increasingly become larger, the need to optimize queries for the Vertipaq engine that underpins Power BI, Power Pivot and Microsoft Analysis Services.
When a variable is calculated in a DAX expression, its value is computed only once. This means that if you use the variable multiple times in your formula, Power BI doesn’t need to recalculate it each time. This reduces the total number of computations required to execute the formula. The less compute resources need for a calculation equates to a more responsive report.
Improved Query Plan
Variables can also help the DAX engine to create a more efficient query plan. By breaking down a complex calculation into smaller, more manageable parts, the DAX engine can optimize how it processes each part of the calculation.
There is an entire world of query optimization with DAX to dig into, Microsoft publishes a guide to optimizing variables to improve your DAX formulas. We highly recommend giving it a read if you frequently work with larger datasets or find that your report is running slow when users interact with visual elements.
How to Write Variables in DAX
Writing a variable in DAX involves two parts, declaration and return. The declaration step gives a nickname or alias to part of the expression. The return step tells Power BI that you will be using the newly named variables as part of a formula, and asks it to return the results.
The syntax is as follows:
VAR VariableName = Expression
Your DAX formula using the variable
And here is an example of using Variables in a formula.
VAR TotalSales = SUM(Sales[Amount])
VAR NumberOfSales = COUNTROWS(Sales)
TotalSales / NumberOfSales
NumberOfSales are variables that store the total sales amount and the number of sales, respectively. They’re then inserted into a formula where the variable expressions are passed through.
It’s a similar concept to creating separate measures and nesting them together, but everything is done within a single formula or DAX measure.
The Difference Between Variables and Measure in Power BI
People new to Power BI often get variables and measures mixed up. While they’re related, they two are distinctly different. Measures contain DAX formula.
- Variable: A local named expression within a DAX formula. It’s not reusable outside the formula where it’s defined.
- Measure: A reusable calculation used across different reports and visualizations in Power BI. It’s a global calculation within the model.
For instance, a measure might calculate total sales across multiple reports, while a variable could be used within a measure to hold an intermediate calculation specific to that measure.
Variables in Power BI’s DAX can save you a lot of time. They help simplify and clarify complex formulas. Plus, they’re great for boosting the speed of your queries. This means when people are looking through your reports, everything runs more smoothly.
Measures in Power BI are built using DAX formulas, and variables can be part of these formulas. However, it’s important to note that you don’t always have to use variables in your measures.