- A
SELECT session_id, ARRAY_AGG(event_time ORDER BY event_time LIMIT 1) FROM sessions GROUP BY session_id
Why wrong: Incorrect: ARRAY_AGG with LIMIT 1 is unnecessary and less efficient than a simple MIN.
- B
SELECT a.session_id, a.event_time FROM sessions a INNER JOIN (SELECT session_id, MIN(event_time) min_ts FROM sessions GROUP BY session_id) b ON a.session_id = b.session_id AND a.event_time = b.min_ts
Why wrong: Incorrect: Self-join with subquery doubles the table scan and is inefficient compared to a simple GROUP BY.
- C
SELECT session_id, MIN(event_time) FROM sessions GROUP BY session_id
Correct: Uses MIN and GROUP BY, which is the most efficient method for retrieving the first event time per session.
- D
SELECT session_id, event_time FROM sessions QUALIFY ROW_NUMBER() OVER (PARTITION BY session_id ORDER BY event_time) = 1
Why wrong: Incorrect: Although it returns the correct result, using QUALIFY and ROW_NUMBER adds unnecessary overhead (window function with sorting) for this simple aggregation.
PCDE MIN aggregate function Practice Question
This PCDE practice question tests your understanding of define data structures and implement sql for business intelligence. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: mIN aggregate function. 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.
A BI team needs to analyze user behavior with sessionization. Each event has a timestamp and session ID. The table 'sessions' contains columns: session_id, user_id, event_time, event_name. The team wants the first event time per session. Which query is most efficient?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"first"Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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
SELECT session_id, MIN(event_time) FROM sessions GROUP BY session_id
Option C is the most efficient query for this requirement. It uses the MIN aggregate function with GROUP BY, which leverages the database's optimized aggregation engine. This approach requires scanning the table once and grouping by session_id, computing the minimum event_time per group. It is both concise and performant. Option D with QUALIFY and ROW_NUMBER is more complex and may be less efficient because it involves window function processing and sorting within each partition, which is unnecessary when only the minimum time is needed.
Key principle: MIN aggregate function
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
SELECT session_id, ARRAY_AGG(event_time ORDER BY event_time LIMIT 1) FROM sessions GROUP BY session_id
Why it's wrong here
Incorrect: ARRAY_AGG with LIMIT 1 is unnecessary and less efficient than a simple MIN.
- ✗
SELECT a.session_id, a.event_time FROM sessions a INNER JOIN (SELECT session_id, MIN(event_time) min_ts FROM sessions GROUP BY session_id) b ON a.session_id = b.session_id AND a.event_time = b.min_ts
Why it's wrong here
Incorrect: Self-join with subquery doubles the table scan and is inefficient compared to a simple GROUP BY.
- ✓
SELECT session_id, MIN(event_time) FROM sessions GROUP BY session_id
Why this is correct
Correct: Uses MIN and GROUP BY, which is the most efficient method for retrieving the first event time per session.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
MIN aggregate function
- ✗
SELECT session_id, event_time FROM sessions QUALIFY ROW_NUMBER() OVER (PARTITION BY session_id ORDER BY event_time) = 1
Why it's wrong here
Incorrect: Although it returns the correct result, using QUALIFY and ROW_NUMBER adds unnecessary overhead (window function with sorting) for this simple aggregation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many candidates assume that using QUALIFY with ROW_NUMBER is the standard modern pattern for sessionization, but for the simple requirement of retrieving the first event time per session, a straightforward GROUP BY with MIN is more efficient and is the recommended approach in BigQuery and similar platforms. The trap is over-engineering the solution.
Detailed technical explanation
How to think about this question
The QUALIFY clause filters the results of a window function after it has been computed, similar to HAVING for aggregate functions. Under the hood, ROW_NUMBER() assigns a unique sequential integer to each row within a partition, and QUALIFY removes all rows where the row number is not 1, avoiding the need for a subquery or self-join. This is particularly useful in Snowflake and BigQuery, where QUALIFY is natively supported and can be more efficient than alternative methods, especially on large datasets.
KKey Concepts to Remember
- MIN aggregate function
- GROUP BY clause
- Aggregate query efficiency
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
MIN aggregate function
Real-world example
How this comes up in practice
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. MIN aggregate function Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
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FAQ
Questions learners often ask
What does this PCDE question test?
Define data structures and implement SQL for Business Intelligence — This question tests Define data structures and implement SQL for Business Intelligence — MIN aggregate function.
What is the correct answer to this question?
The correct answer is: SELECT session_id, MIN(event_time) FROM sessions GROUP BY session_id — Option C is the most efficient query for this requirement. It uses the MIN aggregate function with GROUP BY, which leverages the database's optimized aggregation engine. This approach requires scanning the table once and grouping by session_id, computing the minimum event_time per group. It is both concise and performant. Option D with QUALIFY and ROW_NUMBER is more complex and may be less efficient because it involves window function processing and sorting within each partition, which is unnecessary when only the minimum time is needed.
What should I do if I get this PCDE question wrong?
Review mIN aggregate function, then practise related PCDE questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
MIN aggregate function
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Last reviewed: Jun 30, 2026
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