The answer is a delimiter mismatch, as some files may use a different delimiter like a tab instead of a comma, causing Athena to parse those rows incorrectly and return zero rows from those files. This happens because the SerDe in the table definition expects comma-delimited values, so when files with a different delimiter are read, the rows fail to parse and are silently skipped, leading to a count discrepancy where SELECT COUNT(*) returns fewer rows than the number of data files in S3. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this scenario tests your understanding of how Athena’s schema-on-read works and how SerDe configurations directly affect query results—a common trap is assuming all files in the same S3 location share the same format. Remember the memory tip: “Delimiter mismatch drops data; check your SerDe before you count.”
MLS-C01 Exploratory Data Analysis Practice Question
This MLS-C01 practice question tests your understanding of exploratory data analysis. 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.
Network Topology
Refer to the exhibit. A data scientist queries the table with 'SELECT COUNT(*) FROM mytable' in Athena and gets a result of 1000 rows. However, the scientist knows there are 1500 data files in the S3 location. What is the most likely reason for the discrepancy?
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
✓
Some files may use a different delimiter (e.g., tab) and are not parsed correctly, resulting in zero rows from those files.
The table is not partitioned (empty PartitionKeys), and the SerDe expects comma-delimited files. If some files use a different delimiter (e.g., tab), those rows may not be parsed correctly, leading to fewer rows counted. Option A is wrong because the table schema matches the data. Option B is wrong because Athena can handle many small files, but count should still include all rows. Option D is wrong because Athena does not skip files based on size unless explicitly filtered.
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.
✓
Some files may use a different delimiter (e.g., tab) and are not parsed correctly, resulting in zero rows from those files.
Why this is correct
If delimiter is not comma, rows are not parsed, reducing count.
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.
✗
The table schema does not match the data, causing some files to be skipped.
Why it's wrong here
Schema matches; all columns are present.
✗
Some files may be empty or contain only headers, so they contribute 0 rows.
Why it's wrong here
Empty files contribute 0 rows but do not cause missing rows from non-empty files.
✗
Athena skips files larger than a certain size to prevent scanning too much data.
Why it's wrong here
Athena does not skip large files; it scans all files.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Identify which MLS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Exploratory Data Analysis — This question tests Exploratory Data Analysis — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Some files may use a different delimiter (e.g., tab) and are not parsed correctly, resulting in zero rows from those files. — The table is not partitioned (empty PartitionKeys), and the SerDe expects comma-delimited files. If some files use a different delimiter (e.g., tab), those rows may not be parsed correctly, leading to fewer rows counted. Option A is wrong because the table schema matches the data. Option B is wrong because Athena can handle many small files, but count should still include all rows. Option D is wrong because Athena does not skip files based on size unless explicitly filtered.
What should I do if I get this MLS-C01 question wrong?
Identify which MLS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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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