Power BI Interview Questions and Answers for experienced

1. What are the key differences between Power BI Desktop and Power BI Service?

   • Answer: Power BI Desktop is a free application used for creating and editing Power BI reports and dashboards. Power BI Service is a cloud-based service where you publish and share your reports, collaborate with others, and access data visualizations. Desktop focuses on development, while the Service focuses on sharing and collaboration. Desktop doesn't include features like scheduled data refreshes or row-level security, which are integral to the Service.

2. Explain the different types of data sources that Power BI can connect to.

   • Answer: Power BI supports a wide range of data sources, including relational databases (SQL Server, Oracle, MySQL), cloud databases (Azure SQL Database, Snowflake, Google BigQuery), NoSQL databases (MongoDB), Excel files, CSV files, text files, web services (OData, REST APIs), and many more. It also connects to on-premise data sources via gateways and supports various cloud storage services (Azure Blob Storage, OneDrive, SharePoint).

3. What are DAX measures, and how are they different from calculated columns?

   • Answer: DAX measures are calculations that are dynamic and recalculate whenever the underlying data or filters change. Calculated columns are static; they're computed once when the data model is processed and don't automatically recalculate with every filter change. Measures are ideal for interactive analysis, while calculated columns are suitable for pre-calculated values that don't need frequent recalculation.

4. Describe the different types of relationships that can be established in Power BI Data Model.

   • Answer: Power BI supports one-to-one, one-to-many, and many-to-many relationships. One-to-one means one record in a table relates to only one record in another table. One-to-many is the most common, where one record in a table can relate to multiple records in another. Many-to-many relationships require an intermediary join table (often created automatically by Power BI).

5. Explain the concept of Data Modeling in Power BI. Why is it important?

   • Answer: Data modeling involves designing the structure of your data within Power BI, defining relationships between tables, and optimizing the model for performance and efficiency. A well-designed data model is crucial for accurate, efficient, and insightful reporting. Poor data modeling can lead to incorrect calculations, slow performance, and difficulty in understanding the data.

6. What are the different visualization types available in Power BI, and when would you use each?

   • Answer: Power BI offers a wide array of visualizations, including tables, matrices, charts (bar, line, pie, scatter), maps, cards, gauges, and more. The choice depends on the type of data and the insights you want to convey. For example, bar charts are good for comparisons, line charts for trends, and maps for geographical data.

7. How do you handle large datasets in Power BI?

   • Answer: Techniques for handling large datasets include data sampling, data aggregation (summarizing data before loading it into Power BI), using DirectQuery mode (querying the data source directly instead of importing it), optimizing the data model (creating efficient relationships and measures), and utilizing Power BI's built-in performance tools to identify and address bottlenecks.

8. Explain the role of Data Gateways in Power BI.

   • Answer: Data gateways enable Power BI to connect to and refresh data from on-premises data sources. They act as a bridge between the cloud-based Power BI Service and your local network. They are necessary for scheduled data refreshes and for accessing data that is not publicly accessible on the internet.

9. What are Power BI paginated reports? How do they differ from regular Power BI reports?

   • Answer: Paginated reports are designed for precise control over report layout and page breaks, similar to traditional report writing tools. They are ideal for reports that need to be printed or exported to PDF with a specific, pre-defined layout. Regular Power BI reports are interactive and dynamic, adapting to screen size and user interactions.

10. How to create a calculated column using DAX in Power BI? Provide an example.

    • Answer: To create a calculated column, you go to the Modeling tab in Power BI Desktop, then choose "New Column." You then enter a DAX formula. For example, to add a calculated column "Total Sales" based on "Quantity" and "Unit Price" columns, the formula would be: `Total Sales = 'Sales'[Quantity] * 'Sales'[Unit Price]`

11. What is the purpose of the ALL function in DAX? Provide an example.

    • Answer: The ALL function removes all filters from a table or column. It is useful in calculations where you need to compare values against the total, regardless of any applied filters. For example, `Total Sales = SUMX(ALL('Sales'), 'Sales'[Total Sales])` would calculate the total sales across all rows, ignoring any filters on the 'Sales' table.

12. What is the difference between a slicer and a filter in Power BI?

    • Answer: Both slicers and filters control which data is displayed in a report. Slicers are interactive visual controls (buttons, dropdowns) that allow users to select specific values to filter the data. Filters are typically applied behind the scenes, either through the filter pane or within DAX measures, and may not be as visibly interactive to the end-user.

13. Explain the concept of Row Level Security (RLS) in Power BI.

    • Answer: RLS allows you to control which rows of data a user can see based on their identity. It uses DAX expressions to define security rules that determine data visibility at the row level, enhancing data security and ensuring data confidentiality.

14. What are the different data refresh modes in Power BI?

    • Answer: Power BI offers "Import," "DirectQuery," and "Composite" data refresh modes. Import mode loads data into the Power BI model; DirectQuery queries the data source directly; Composite combines aspects of both, importing some data and querying others directly.

15. How do you create a Power BI report from an Excel file?

    • Answer: In Power BI Desktop, go to "Get Data," select "Excel," browse to your file, and import or connect to the data. Power BI will detect the tables within the Excel file and allow you to import them into the data model.

16. How to publish a Power BI report to the Power BI service?

    • Answer: In Power BI Desktop, click "Publish" and sign in to your Power BI account. Power BI will upload your report to your Power BI workspace.

17. What are some best practices for designing effective Power BI dashboards?

    • Answer: Best practices include focusing on key performance indicators (KPIs), using clear and concise visualizations, maintaining a consistent design style, ensuring data accuracy, and making the dashboard easily understandable and actionable for the intended audience.

18. How do you handle errors and exceptions in DAX formulas?

    • Answer: DAX functions like `IFERROR` and `ISERROR` help manage errors. `IFERROR` returns a specified value if an error occurs, while `ISERROR` checks if a value is an error. Proper data cleansing and validation before importing data into Power BI can also significantly reduce errors.

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