The correct answer is that the join column in the first table contains values that do not exist in the second table. This error occurs because Power Query’s merge operation performs a lookup on the second table using each value from the first table’s join column; when a value is absent in the second table, the engine cannot find a matching row, triggering the “key didn’t match any rows” message. On the Microsoft Power BI Data Analyst PL-300 exam, this scenario tests your understanding of join types and data integrity—specifically that an inner or left outer merge requires all keys from the primary table to exist in the lookup table. A common trap is misdiagnosing this as a data type mismatch or a duplicate key issue, but the core problem is simply missing values. Remember the mnemonic: “No match, no row—check your first table’s key list for orphans.”
PL-300 Prepare the data Practice Question
This PL-300 practice question tests your understanding of prepare the data. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 receive a Power Query error: 'Expression.Error: The key didn't match any rows in the table.' This occurs when merging two queries. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The join column in the first table contains values that do not exist in the second table.
Option D is correct because the error 'The key didn't match any rows in the table' occurs during a merge operation when Power Query attempts to find a matching value from the first table's join column in the second table's join column, but no match exists. This is a standard behavior for inner joins or left outer joins where the lookup fails, and it typically indicates that the first table contains values absent in the second table.
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.
✗
The join columns have different data types.
Why it's wrong here
Data type mismatch typically causes a type conversion error, not a missing key error.
✗
The second table is empty due to a permission issue.
Why it's wrong here
An empty table would cause a different error or no rows, but the specific error indicates key mismatch.
✗
The join columns contain duplicate values.
Why it's wrong here
Duplicate keys cause multiple matches, not missing keys.
✓
The join column in the first table contains values that do not exist in the second table.
Why this is correct
The error 'key didn't match any rows' means the merge operation found no matching rows for some keys.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Microsoft often tests the misconception that this error is caused by data type mismatches or duplicate values, but the actual cause is a missing key in the lookup table, which is a fundamental concept in Power Query merge operations.
Detailed technical explanation
How to think about this question
Under the hood, Power Query's Table.Join or Merge operation uses a hash-based lookup to match keys; when a key from the first table is not found in the second table's hash table, it raises the 'key didn't match any rows' error only if the join type is set to 'Left Outer' or 'Full Outer' and the 'Use fuzzy matching' option is disabled. In real-world scenarios, this often happens when merging customer IDs from a sales table with a customer master table where some IDs are missing due to data entry errors or incomplete imports.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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: The join column in the first table contains values that do not exist in the second table. — Option D is correct because the error 'The key didn't match any rows in the table' occurs during a merge operation when Power Query attempts to find a matching value from the first table's join column in the second table's join column, but no match exists. This is a standard behavior for inner joins or left outer joins where the lookup fails, and it typically indicates that the first table contains values absent in the second table.
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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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