The answer is to add a calculated column in Power Query that computes NetAmount = Quantity * UnitPrice - Discount, then create a simple SUM measure. This is correct because pre-calculating columns in Power Query for performance shifts the row-level computation to the data refresh stage, where Power Query’s M engine processes it efficiently and stores the result in the table. In contrast, performing this calculation inside a DAX measure would force a row-by-row iteration at query time, which is far slower and can degrade report responsiveness. On the PL-300 exam, this scenario tests your understanding of where to push heavy transformations—Power Query is optimized for bulk row operations, while DAX excels at aggregating pre-computed values. A common trap is choosing a DAX calculated column or a complex measure that recalculates on the fly, both of which hurt performance. Memory tip: “Refresh to store, query to sum”—let Power Query do the heavy lifting during refresh so your measures stay lean and fast.
PL-300 Model the data Practice Question
This PL-300 practice question tests your understanding of model the data. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A Power BI developer has a fact table that contains sales data at the transaction level. The table includes columns: TransactionID, ProductID, CustomerID, DateKey, Quantity, UnitPrice, Discount, and SalesAmount. The developer wants to create a measure for total sales after discount. Which approach is best for performance and accuracy?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add a calculated column in Power Query: NetAmount = Quantity * UnitPrice - Discount, then create a measure: SUM(Sales[NetAmount])
Option B is correct because it performs the net amount calculation at the row level in Power Query (M), which is computed during data refresh and stored in the table. This avoids runtime row-by-row iteration in DAX, making the measure SUM(Sales[NetAmount]) a simple, highly efficient aggregation. It ensures both performance and accuracy, as the discount is applied per transaction before aggregation.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Create a measure: SUM(Sales[SalesAmount]) - SUM(Sales[Discount])
Why it's wrong here
This subtracts total discount from total sales, not per row.
✓
Add a calculated column in Power Query: NetAmount = Quantity * UnitPrice - Discount, then create a measure: SUM(Sales[NetAmount])
Why this is correct
Calculated columns are computed at refresh time, improving query performance.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Create a measure: SUMX(Sales, Sales[Quantity] * Sales[UnitPrice] - Sales[Discount])
Why it's wrong here
SUMX iterates row by row, which can be slower than a calculated column.
✗
Create a measure: SUM(Sales[Quantity] * Sales[UnitPrice]) - SUM(Sales[Discount])
Why it's wrong here
This incorrectly aggregates before subtracting.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a DAX measure using SUMX or a simple subtraction of aggregated columns is equivalent in performance, but the exam tests the understanding that pre-calculating row-level logic in Power Query (M) is the most performant approach for large fact tables, while also ensuring mathematical accuracy.
Detailed technical explanation
How to think about this question
In Power Query (M), adding a calculated column like NetAmount is materialized during the refresh process and stored in the VertiPaq engine, which compresses it efficiently. This contrasts with DAX calculated columns or measures that use SUMX, which are evaluated at query time and can cause performance degradation due to row context iteration. A real-world scenario where this matters is a fact table with millions of rows—pre-calculating the net amount in Power Query can reduce query response times from seconds to milliseconds.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Model the data — This question tests Model the data — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Add a calculated column in Power Query: NetAmount = Quantity * UnitPrice - Discount, then create a measure: SUM(Sales[NetAmount]) — Option B is correct because it performs the net amount calculation at the row level in Power Query (M), which is computed during data refresh and stored in the table. This avoids runtime row-by-row iteration in DAX, making the measure SUM(Sales[NetAmount]) a simple, highly efficient aggregation. It ensures both performance and accuracy, as the discount is applied per transaction before aggregation.
What should I do if I get this PL-300 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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