How to Use Left Outer Join in Power Query
You need to combine two tables from different sources: 'Orders' from SQL Server and 'Returns' from an Excel file. Both tables have a column named 'OrderID'. You want to include all orders and only matching returns. Which join type should you use in Power Query?
Quick Answer
The answer is Left Outer Join. This join type is correct because it returns all rows from the first table, 'Orders', and only the matching rows from the second table, 'Returns', based on the 'OrderID' column, ensuring no order is dropped even if it has no corresponding return. In the context of the Microsoft Power BI Data Analyst PL-300 exam, this question tests your understanding of table merging in Power Query, specifically how to preserve the left table’s integrity while enriching it with related data from a right table. A common trap is confusing Left Outer with Inner Join, which would exclude orders without returns. To remember, think of the left outer join as “keep everything on the left, add matches from the right”—the left table is the boss, and the right table is optional.
⚠ Common exam trap
Many candidates confuse Left Outer Join with Right Outer Join, mistakenly thinking they need to include all returns instead of all orders, or they default to Inner Join without considering the requirement to preserve unmatched rows from the left table.
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
✓
Left Outer Join
In Power Query, a Left Outer Join returns all rows from the first (left) table ('Orders') and only the matching rows from the second (right) table ('Returns'), based on the 'OrderID' column. This matches the requirement to include all orders and only matching returns, ensuring no order is dropped even if it has no corresponding return.
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 it's wrong here
Inner join only returns rows where there is a match in both tables.
- ✗
Right Outer Join
Why it's wrong here
Right outer join returns all rows from the second table (Returns) and matching from Orders.
- ✗
Full Outer Join
Why it's wrong here
Full outer join returns all rows from both tables.
- ✓
Left Outer Join
Why this is correct
Left outer join returns all rows from the first table and matching rows from the second.
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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 need to combine two tables: Sales and Products, where Sales has a ProductID column and Products has a ProductKey column. The tables have a many-to-one relationship. Which Power Query transformation should you use?
easy- A.Group By.
- B.Append Queries.
- ✓ C.Merge Queries.
- D.Pivot Column.
Why C: Merge Queries (C) is correct because it performs a join between two tables based on matching columns, which is exactly what is needed to combine Sales and Products using ProductID and ProductKey. This transformation supports many-to-one relationships and allows you to expand related columns from the Products table into the Sales table, enabling further data analysis.
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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.