Microsoft Power BI Power BI Data Modeling 2 — Questions and Answers
Question 1: In Power BI, what is a 'calculated column' and when is it evaluated?
- A column created in Power Query, evaluated during refresh
- A DAX column added to a table, evaluated at data refresh/load time (Correct answer)
- A measure stored in a column, evaluated at query time
- A column from a SQL view, evaluated at runtime
Correct answer: A DAX column added to a table, evaluated at data refresh/load time
A calculated column is defined using DAX and is computed and stored in the model at data refresh or load time.
Question 2: What is a 'snowflake schema' in Power BI?
- A schema optimized for cold climates
- A star schema where dimension tables are further normalized into sub-tables (Correct answer)
- A schema with no fact tables
- A schema with only date dimensions
Correct answer: A star schema where dimension tables are further normalized into sub-tables
A snowflake schema extends a star schema by normalizing dimension tables into multiple related sub-dimension tables.
Question 3: When should you use a 'measure' instead of a 'calculated column' in Power BI?
- When you need to store the result in the table
- When you need a row-by-row calculation stored at refresh
- When the calculation aggregates data dynamically based on filter context (Correct answer)
- When you want to use the result in a slicer
Correct answer: When the calculation aggregates data dynamically based on filter context
Measures are best for dynamic aggregations that change based on the current filter or row context at query time.
Question 4: What happens to model performance if you have too many calculated columns in a large Power BI dataset?
- Performance improves due to pre-computation
- Dataset file size increases and refresh times slow down (Correct answer)
- Measures become invalid
- Relationships break automatically
Correct answer: Dataset file size increases and refresh times slow down
Each calculated column adds to the in-memory storage of the model, increasing file size and potentially slowing refresh times.
Question 5: What is the purpose of a 'date table' in a Power BI data model?
- To store future forecast dates
- To enable time intelligence DAX functions to work correctly (Correct answer)
- To replace the calendar in Power Query
- To display dates in report visuals only
Correct answer: To enable time intelligence DAX functions to work correctly
A properly configured date table is required for DAX time intelligence functions like TOTALYTD and SAMEPERIODLASTYEAR to work correctly.
Question 6: How do you mark a table as a 'Date Table' in Power BI Desktop?
- Right-click in Report View and select Date Table
- In Model View, select Mark as Date Table from the table's context menu (Correct answer)
- Add a calculated column named 'IsDateTable'
- Enable it in Power Query's date settings
Correct answer: In Model View, select Mark as Date Table from the table's context menu
You right-click the table in Model View (or use the Table tools ribbon) and select 'Mark as date table' to designate it as the official date table.
Microsoft Power BI Power BI Data Modeling 2
In Power BI, what is a 'calculated column' and when is it evaluated?