Write a query to find time to first action from transactions
Last updated: April 16, 2026
Quick Overview
Write a SQL query to calculate time to first action from the transactions table, considering nulls and duplicates.
Amazon
April 16, 20262
4
4,647 solved
Write a SQL query to calculate time to first action from the transactions table, considering nulls and duplicates.
This question from Amazon's Take-home Project tests practical data skills. The interviewer wants to see clean, efficient queries that handle edge cases like NULLs, duplicates, and large datasets.
What the Interviewer Expects
- Use advanced SQL features: window functions, CTEs, subqueries
- Write efficient queries that avoid common performance pitfalls
- Handle complex data transformations with multiple joins and aggregations
- Discuss indexing strategy and query optimization
- Address data quality issues: duplicates, missing values, outliers
Key Topics to Cover
How to Approach This
- Clarify the schema and expected output format before writing queries.
- Use CTEs (WITH clauses) to break complex queries into readable steps.
- Consider window functions (ROW_NUMBER, RANK, LAG, LEAD) for ranking and sequential analysis.
- Watch for NULLs, duplicates, and edge cases in JOINs and GROUP BY.
- For pandas, prefer vectorized operations over row-by-row iteration.
Possible Follow-up Questions
- What indexes would you create to support this query?
- What would you do if this query needs to run every 5 minutes?
- Can you rewrite this without using subqueries?
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Practice SQL ProblemsSample Answer
Problem Understanding
The data involved in this query is from the transactions table, which contains records of transactions including timestamps of actions taken by users. The goal is to calculate the time taken for eac...
Approach
- Identify Relevant Fields: Determine the necessary fields in the
transactionstable, such asuser_id,transaction_time, and any associated action timestamps from anactionstable. - **...
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