Write a query to find cumulative sum from transactions
Last updated: December 30, 2025
Quick Overview
Write a SQL query to calculate cumulative sum from the transactions table, considering nulls and duplicates.
Booking.com
December 30, 202581
4
2,325 solved
Write a SQL query to calculate cumulative sum from the transactions table, considering nulls and duplicates.
Booking.com asks this during the Technical Screen because data engineering skills are critical for the role. You should be comfortable with complex joins, window functions, CTEs, and performance optimization.
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?
- How would you handle slowly changing dimensions in this scenario?
- Can you rewrite this without using subqueries?
- How would you optimize this query for a table with 100 million rows?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task is to calculate the cumulative sum of transaction amounts from the transactions table, taking into account potential NULL values and duplicate transaction records. The transactions table ...
Approach
- Identify the Data: We need to focus on the
amountfield for cumulative calculations and thetransaction_datefor ordering. - Handle Duplicates: We should ensure that duplicates are ha...
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