Creating dataflows in Power BI - Formula Evaluation Walkthrough

Sample Data

Sample customer data table with IDs, names, and countries to be used in a Power BI dataflow.

Cell	Value
A1	CustomerID
B1	Name
C1	Country
A2	101
B2	Alice
C2	USA
A3	102
B3	Bob
C3	Canada
A4	103
B4	Charlie
C4	USA

Formula Trace

Table.SelectRows(Source, each [Country] = "USA")

Step 1: Source

Step 2: each [Country] = "USA"

Step 3: Table.SelectRows(Source, each [Country] = "USA")

Cell Reference Map

    A          B          C
1 CustomerID   Name      Country
2 101         Alice      USA  <-- included
3 102         Bob       Canada
4 103         Charlie    USA  <-- included

The formula references the 'Country' column in the source table to filter rows where Country is 'USA'. Rows 2 and 4 meet this condition.

Result

    A          B          C
1 CustomerID   Name      Country
2 101         Alice      USA
3 103         Charlie    USA

The resulting table after applying the filter shows only customers from the USA.

Sheet Trace Quiz - 3 Questions

What does the formula Table.SelectRows(Source, each [Country] = "USA") do?

A Adds a new column named USA

B Deletes rows where Country is USA

C Keeps only rows where Country is USA

D Sorts the table by Country

Key Result

Table.SelectRows filters a table by keeping rows that meet a condition.

Practice

1. What is the main purpose of creating a dataflow in Power BI?

easy

A. To create visual charts and graphs

B. To prepare and clean data once for use in multiple reports

C. To write DAX formulas for calculations

D. To schedule report email delivery

Solution

1. Step 1: Understand dataflow purpose

   Dataflows are designed to prepare and clean data centrally.

2. Step 2: Compare options

   Creating visuals and writing DAX are report tasks, not dataflow tasks.

3. Final Answer:

   To prepare and clean data once for use in multiple reports -> Option B

4. Quick Check:

   Dataflows centralize data prep = A [OK]

Hint: Dataflows focus on data prep, not visuals or formulas [OK]

Common Mistakes:

• Confusing dataflows with report visuals
• Thinking dataflows create charts
• Assuming dataflows handle email scheduling

2. Which of the following is the correct first step to create a dataflow in Power BI Service?

easy

A. Create a new report and add visuals

B. Open Power BI Desktop and click 'Get Data'

C. Go to workspace > New > Dataflow

D. Write a DAX measure in the report

Solution

1. Step 1: Identify dataflow creation location

   Dataflows are created in Power BI Service under a workspace.

2. Step 2: Verify correct menu path

   The correct path is Workspace > New > Dataflow, not Power BI Desktop or report creation.

3. Final Answer:

   Go to workspace > New > Dataflow -> Option C

4. Quick Check:

   Create dataflow in service workspace = C [OK]

Hint: Dataflows start in Power BI Service workspace, not Desktop [OK]

Common Mistakes:

• Trying to create dataflows in Power BI Desktop
• Confusing dataflows with reports
• Starting with DAX instead of data preparation

3. Consider this scenario: You create a dataflow that connects to an Excel file and applies a filter to keep only rows where Sales > 1000. What will happen when you refresh the dataflow?

medium

A. The dataflow will load all rows from Excel without filtering

B. The dataflow will refresh only if the Excel file is open

C. The dataflow will fail because filters are not allowed

D. The dataflow will load only rows where Sales > 1000

Solution

1. Step 1: Understand dataflow refresh behavior

   Dataflows apply all transformations including filters during refresh.

2. Step 2: Apply filter effect

   Only rows meeting Sales > 1000 condition are loaded into the dataflow.

3. Final Answer:

   The dataflow will load only rows where Sales > 1000 -> Option D

4. Quick Check:

   Filters apply on refresh = B [OK]

Hint: Filters in dataflows apply automatically on refresh [OK]

Common Mistakes:

• Thinking filters don't apply in dataflows
• Assuming dataflows need source files open
• Believing dataflows load all data regardless of filters

4. You created a dataflow but it fails to refresh with an error saying "Invalid credentials". What is the most likely cause and fix?

medium

A. The data source credentials are missing or expired; update credentials in dataflow settings

B. The dataflow name is invalid; rename the dataflow

C. The Power BI Service is down; wait and retry later

D. The dataflow has too many columns; reduce columns

Solution

1. Step 1: Analyze error message

   "Invalid credentials" means Power BI cannot access the data source due to authentication issues.

2. Step 2: Identify fix

   Updating or re-entering credentials in dataflow settings resolves this issue.

3. Final Answer:

   The data source credentials are missing or expired; update credentials in dataflow settings -> Option A

4. Quick Check:

   Invalid credentials = update data source login [OK]

Hint: Check and update data source credentials when refresh fails [OK]

Common Mistakes:

• Ignoring credential errors and blaming dataflow name
• Assuming service outage without checking credentials
• Thinking column count causes credential errors

5. You want to create a dataflow that combines data from two different SQL databases and refreshes daily. Which approach is best to ensure efficient and maintainable dataflows?

hard

A. Create two separate dataflows for each database, then create a third dataflow to merge their outputs

B. Create one dataflow that connects to both databases and merges data in a single query

C. Create one dataflow per database and merge data only in Power BI Desktop reports

D. Create multiple reports instead of dataflows to handle merging

Solution

1. Step 1: Consider modular design

   Separating dataflows by source improves maintainability and reusability.

2. Step 2: Combine dataflows for merging

   Creating a third dataflow to merge outputs centralizes transformation and refresh logic.

3. Final Answer:

   Create two separate dataflows for each database, then create a third dataflow to merge their outputs -> Option A

4. Quick Check:

   Modular dataflows + merge = A [OK]

Hint: Split sources, then merge in a separate dataflow for clarity [OK]

Common Mistakes:

• Merging different sources in one complex dataflow
• Merging only in reports causing repeated work
• Avoiding modular dataflows for easier updates