PL-300 Prepare the data Practice Question
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?
⚠ Common exam trap
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.
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
✓
The join column in the first table contains values that do not exist in the second table.
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.
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 mismatches in Power Query merge operations produce a conversion-related error (e.g., 'We couldn't convert Value to Text') before any row matching occurs. The query engine must first coerce both join columns to a common type, so a mismatch manifests as a type conversion failure, not as a missing key.
- ✗
The second table is empty due to a permission issue.
Why it's wrong here
If the second table were empty because of permission issues, you would see a credential or access error during the data load step, or the merge would simply output zero matches. The 'key didn't match any rows' error is thrown for specific key values that cannot be found, implying the second table has rows, but those particular keys are absent.
- ✗
The join columns contain duplicate values.
Why it's wrong here
Duplicate values in join columns do not cause missing-key errors; they cause fan-out, where multiple rows from one table match multiple rows from the other. Power Query will return a Cartesian product for each duplicate group or add nulls for unmatched rows, depending on the join kind, but it never reports a key that didn't match.
- ✓
The join column in the first table contains values that do not exist in the second table.
Why this is correct
This is the direct reason for the error: when a value in the first table's join column is not present in the second table's join column, the merge operation cannot find a matching row for that key. If the join kind requires a match or you are using a lookup-style operation, Power Query raises 'Expression.Error: The key didn't match any rows' instead of silently inserting nulls.
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