PL-300 Prepare the data Practice Question
You are merging two tables in Power Query: 'Orders' and 'Customers'. You want to include only rows from Orders that have a matching CustomerID in Customers. Which join kind should you use?
⚠ Common exam trap
Many candidates confuse left outer join (which keeps all rows from the first table) with inner join, mistakenly thinking they need to preserve all Orders rows, but the question explicitly requires only matching rows.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Inner join
An inner join in Power Query returns only rows from both tables where there is a match on the join key. Since you want to include only rows from Orders that have a matching CustomerID in Customers, the inner join is the correct choice. It filters out any Orders rows without a corresponding CustomerID in the Customers table.
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 join
Why this is correct
Inner join is the correct choice because it returns only rows where the join key exists in both the Orders and Customers tables. This effectively filters the data to matching records, which is typically the desired outcome when merging transactional data with dimension tables. In Power Query, inner join is the default and guarantees that no unmatched rows are included, ensuring referential integrity in the merged result.
- ✗
Full Outer join
Why it's wrong here
Full Outer join is incorrect because it retains all rows from both the Orders and Customers tables, creating a union of both datasets. Any order without a matching customer, and any customer without an order, will appear with null values in the opposite table's columns. This inflates the result set with non-matching rows, which contradicts the requirement to return only matching records and can lead to duplicates or unexpected nulls in downstream analysis.
- ✗
Right Outer join
Why it's wrong here
Right Outer join is incorrect because it preserves every row from the right table (Customers) while only including matching rows from the left table (Orders). This means customers who have never placed an order will still appear, with null order data, which dilutes the focus on actual orders. Since the merge is meant to link orders to customers, prioritizing all customer rows skews the output toward customers rather than the orders that need matching.
- ✗
Left Outer join
Why it's wrong here
Left Outer join is incorrect because it keeps all rows from the left table (Orders) and only adds matching customer information, leaving nulls for any order that lacks a corresponding customer. This would include orphaned orders or those with invalid customer IDs, which should be excluded if the goal is to analyze only orders with valid customer relationships. Unlike inner join, it does not enforce the requirement that both sides must match, thus failing to filter out incomplete data.
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Same concept, more angles
1 more way this is tested on PL-300
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. You are merging two queries in Power Query: 'Orders' and 'Customers'. The 'Orders' table has a 'CustomerID' column, and 'Customers' has 'CustomerID' and 'Name'. You need to bring the 'Name' into 'Orders' but only for matching CustomerIDs; unmatched rows should be removed. Which join kind should you use?
hard- A.Right Anti
- B.Full Outer
- ✓ C.Inner
- D.Left Outer
Why C: The Inner join kind in Power Query returns only rows where there is a match in both tables based on the key columns. Since the requirement is to bring the 'Name' into 'Orders' only for matching CustomerIDs and to remove unmatched rows, the Inner join is the correct choice. It ensures that only orders with a corresponding customer in the 'Customers' table are retained, and the 'Name' column is added to those matching rows.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PL-300 practice question is part of Courseiva's free Microsoft certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PL-300 exam.