Power BI Interview Questions – Query
1. [Asked in Deloitte] What is Power Query, and why is it useful in Power BI?
Answer:
Power Query is a powerful data transformation and cleaning tool in Power BI. It helps you import data from various sources, clean it by removing errors or duplicates, transform data formats, and prepare it for visualization. This makes sure your reports use accurate and consistent data.
2. [Asked in Infosys] How do you access Power Query in Power BI Desktop?
Answer:
You can access Power Query by clicking on the “Transform Data” button on the Home tab in Power BI Desktop. This opens a new window called the Power Query Editor, where you can manage data transformations.
3. [Asked in Accenture] Can you name some common data transformations done using Power Query?
Answer:
Common transformations include:
- Removing duplicates and errors.
- Splitting and merging columns.
- Changing data types (text, number, date).
- Filtering rows.
- Adding or removing columns.
- Combining multiple data tables.
4. [Asked in Capgemini] What is the difference between “Merge Queries” and “Append Queries” in Power Query?
Answer:
- Merge Queries combines two tables horizontally (side-by-side) based on matching columns (like a JOIN in SQL).
- Append Queries combines tables vertically (stacking rows), creating a single, longer table.
5. [Asked in TCS] Is it possible to undo a step in Power Query? If so, how?
Answer:
Yes. In Power Query Editor, each action you perform creates a step listed under the “Applied Steps” pane. You can easily undo by removing or editing these steps.
6. [Asked in IBM] What is Query Folding in Power Query, and why is it important?
Answer:
Query Folding means that transformations performed in Power Query are converted into SQL and executed directly at the source database. It improves performance by reducing data load times and server memory usage, especially with large datasets.
7. [Asked in Amazon] How can you remove duplicates from data using Power Query?
Answer:
In the Power Query Editor:
- Select the column(s).
- Right-click and choose “Remove Duplicates”. Power Query automatically removes any duplicate rows based on the selected column.
8. [Asked in Wipro] Can you connect multiple data sources in one Power Query model?
Answer:
Yes, Power Query supports connecting multiple sources (like Excel, databases, CSV files, or web APIs) in one model. You can easily merge or append this data to create a unified dataset.
9. [Asked in Microsoft] What is the M language in Power Query?
Answer:
“M” is the formula language behind Power Query. It’s used to create custom transformations and advanced queries. Each step in the Power Query Editor generates an M code automatically, which can be viewed or edited for advanced data preparation.
10. [Asked in Cognizant] How do you handle errors in data using Power Query?
Answer:
Power Query allows you to handle errors easily by:
- Replacing errors with custom values.
- Removing rows with errors.
- Filtering out error values. You can find these options by right-clicking the column header and selecting the appropriate error-handling method.
