MLS-C01 COUNT(*) Practice Question
Network Topology
The exhibit shows an Athena query result from a table. What is the output of the query?
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
✓
2, 4, 4
The query returns COUNT(col2)=2 (only rows 1 and 3 have non-null col2), COUNT(*)=4 (total rows), COUNT(DISTINCT col1)=4 (distinct values A, B, C, D). Option A is wrong because COUNT(col2) is 2, not 3. Option B is wrong because COUNT(*) is 4, not 3. Option D is wrong because COUNT(DISTINCT col1) is 4, not 3.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
3, 3, 4
Why it's wrong here
COUNT(col2) is 2, not 3; COUNT(*) is 4, not 3.
- ✗
2, 3, 3
Why it's wrong here
COUNT(*) is 4, not 3; distinct col1 is 4, not 3.
- ✓
2, 4, 4
Why this is correct
Correct counts: col2 non-null=2, rows=4, distinct col1=4.
- ✗
2, 4, 3
Why it's wrong here
COUNT(DISTINCT col1) is 4, not 3.
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Same concept, more angles
1 more way this is tested on MLS-C01
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. An Athena query SELECT COUNT(*) FROM table WHERE col1 IS NULL returns the value 5000. What does this value represent?
medium- A.The total number of rows in the table
- ✓ B.The number of rows where col1 is NULL
- C.The number of rows where col1 is not NULL
- D.The number of distinct values in col1
Why B: The exhibit shows the result of an Athena query that counts the number of rows where col1 is NULL. The value 5000 is that count. Therefore, Option B is correct. Option A is incorrect because the query does not count total rows; it filters for NULLs. Option C is incorrect because the query counts NULL rows, not non-NULL. Option D is incorrect because the query does not use DISTINCT to count distinct values.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This MLS-C01 practice question is part of Courseiva's free Amazon Web Services 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 MLS-C01 exam.