The answer is Left Outer, which is the correct join kind for merging the Orders and Customers queries in Power Query. This choice is correct because a Left Outer join retains every row from the first (left) table—Orders—while bringing in matching CustomerName values from the second (right) table, Customers. In a many-to-one relationship, where multiple orders can link to the same customer, this join ensures no order is dropped, and any order without a matching customer simply gets a null for CustomerName. On the Microsoft Power BI Data Analyst PL-300 exam, this scenario tests your understanding of merge operations and join kinds, often appearing in questions about combining fact tables with dimension tables. A common trap is confusing Left Outer with Inner join, which would remove orders missing a customer—something you rarely want in data modeling. Memory tip: think “Left keeps all, right fills in”—the left table is your primary table, and you only add matching data from the right.
PL-300 Prepare the data Practice Question
This PL-300 practice question tests your understanding of prepare 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.
You are merging two queries in Power Query. Query 'Orders' contains columns: OrderID, CustomerID, OrderDate. Query 'Customers' contains columns: CustomerID, CustomerName, Segment. You need to add the CustomerName to the Orders query. The relationship between Orders and Customers is many-to-one. Which join kind should you use?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Left Outer
The goal is to retain all rows from the Orders table while adding CustomerName from the Customers table. A Left Outer join returns all rows from the first (left) table and only matching rows from the second (right) table, filling non-matches with null. Since the relationship is many-to-one, each OrderID may have a matching CustomerID, and you want to keep every order even if a customer is missing — exactly what Left Outer does.
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.
✗
Inner
Why it's wrong here
Inner join would only keep orders that have a matching customer, potentially excluding orders with missing CustomerID.
✓
Left Outer
Why this is correct
Left outer join retains all orders and adds customer name where a match exists; non-matching customers will have null.
Related concept
Read the scenario before looking for a memorised answer.
✗
Right Outer
Why it's wrong here
Right outer join would keep all customers and only orders that match, which is not the requirement.
✗
Full Outer
Why it's wrong here
Full outer join would produce rows for all customers, even those without orders, which is not needed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse Left Outer with Inner join, thinking they must discard non-matching rows to avoid nulls, but the requirement explicitly says to add CustomerName to the Orders query, which implies preserving all orders even if a customer record is missing.
Detailed technical explanation
How to think about this question
Under the hood, Power Query's Left Outer join performs a hash-match on the key columns (CustomerID), preserving the row count of the left table (Orders) and expanding matching columns from the right table. A subtle behavior: if multiple customers share the same CustomerID (violating the many-to-one assumption), Power Query will duplicate rows in the left table for each match, which can cause unexpected row multiplication — always verify key uniqueness. In real-world scenarios, this join is used in star schema data modeling to add dimension attributes to fact tables without losing fact rows.
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
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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.
Prepare the data — This question tests Prepare the data — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Left Outer — The goal is to retain all rows from the Orders table while adding CustomerName from the Customers table. A Left Outer join returns all rows from the first (left) table and only matching rows from the second (right) table, filling non-matches with null. Since the relationship is many-to-one, each OrderID may have a matching CustomerID, and you want to keep every order even if a customer is missing — exactly what Left Outer does.
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.
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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