This document discusses when to transform data for Power BI, emphasizing the choice between SQL, Power Query, and DAX based on 'Roche's Maxim,' which suggests transforming data as far upstream as possible. It outlines the benefits of early transformation, including wider data accessibility and a consistent source for reports, while also recognizing situations where downstream transformation is necessary. The document provides guidance on the appropriate use of each tool for combining data from multiple sources and encourages readers to explore further resources for detailed techniques.

1. Power BI Tips
and Timesavers
When to Transform
Data for Power BI?
SQL or Power Query or DAX

2. Knowing when to transform data for your Power BI
Reports can help you to decide when to use which
technique
SQL, Power Query or DAX
This guide gives you a steer to make the right choice

3. When to transform data for Power BI?
Over the last few weeks, we have shown how data can be combined from multiple sources
We have shown that this can be done in a number of ways using three different tools
But which one do you choose and why?

4. When to transform data for Power BI?
“Data Should be transformed as far upstream as possible, and as far downstream as necessary”
This is called ROCHES MAXIM, by Matthew Roche Principal Program Manager Power BI
Downstream…..Closer to its end destination
Upstream…..Closer to its source

5. Applying Roches Maxim
“Data Should be transformed as far upstream as possible, and as far downstream as necessary”
In simple terms, using our requirement to combine multiple data sources over the last three examples
We should first look to see if we can do it in SQL using the UNION method
If we can’t, then look to do it within Power Query
And lastly, look to using DAX

6. Benefits of transforming early
Transformed data can be made available for wider use cases
For example, a fact table in a data warehouse that has already combined multiple data sources can be used by more users
in more use cases
This means that it only needs to be done once, upstream and controls in place there to ensure its complete and accurate
Data is available on a consistent basis for all users
You get all subsequent reports sharing a common data source and really do have….
One version of the truth

7. Reasons to transform downstream
It's not always possible to perform the transformations upstream
SQL to Power Query
Data could be coming from multiple systems and not from a central data warehouse, it maybe a combination of a SQL
Server database for some parts of the organisation, and a separate Oracle Database for a newly acquired part of the
organisation which is being kept separate
If you had the facility, you could look at creating a simple reporting database to combine these into one, but assuming that
is not an option,
Then you will use the next best option of Power Query to combine these sources using the APPEND functionality

8. Reasons to transform downstream
Power Query or DAX?
Generally, Power Query is better option as its further upstream as the work is done in the data model itself
The data model as an entity can be shared and made available for wider use
This also means that report building is less complex and easier to maintain

9. Reasons to transform downstream
Last but not least DAX
If you need to create a measure that is responsive to slicers
For example
You need to create a card that returns aggregated sales from
multiple sources that are not already combined
You can use the DAX UNION function to combine the data you
need

10. Learn More
You can learn more about Roche’s
Maxim of Data Transformation on
the ssbipolar.com blog
Roche’s Maxim of Data Transformation – BI
Polar (ssbipolar.com)

11. We have outlined the key reasons why you can combine data in multiple ways for
Power BI
When to use SQL
When to use Power Query
And when you need to use a DAX Measure
For more details of any of these specific techniques please visit our Blog

12. For more Tips, Tricks and
Timesavers, visit our website
Business Analytics Blog – Select Distinct
Credit: simon.harrison@selectdistinct.co.uk