I am working with several Excel sheets and a SQL database in Power BI, and I need to consolidate them into a single dataset for my reporting. I am confused between the "Merge" and "Append" functions in Power Query. When should I use one over the other, and is it better to combine them at the data source level or directly within the Power BI relationship view for better model performance?
3 answers
The choice between Merging and Appending depends entirely on your data structure. Think of "Appending" as a vertical operation; you use it when you have tables with the same columns (like Monthly Sales) and you want to stack them into one long table. "Merging" is a horizontal operation, similar to a SQL Join, where you combine tables based on a common column, like joining a "Sales" table with a "Product" table using a ProductID. Generally, it is best practice to perform these operations in Power Query (the backend) rather than using DAX calculated tables. This keeps your data model "thin" and improves the refresh speed of your dashboards.
Are you dealing with a Star Schema design where you have distinct Fact and Dimension tables, or are you trying to flatten everything into one giant flat table for a specific visualization?
You can use the "Folder" data source option if you have multiple CSVs. It automatically appends every file in that folder into one table, which is a huge time saver.
I agree with Margaret. The "Combine Files" feature within the Folder connector is a lifesaver when you get weekly report exports that all share the exact same schema.
Steven, I am currently trying to move toward a Star Schema. However, I’ve noticed that when I merge large tables in Power Query, the "Privacy Levels" warning often pops up and slows down my refresh. Is there a way to bypass this, or is it a sign that I should be doing the heavy lifting—like the Joins—inside my SQL database before the data even reaches Power BI?