Calculate cumulative sum per date
Last updated: September 18, 2025
Quick Overview
Write a query to compute cumulative sum grouped by user, handling edge cases like nulls and duplicates.
PayPal
September 18, 20252
10
831 solved
Write a query to compute cumulative sum grouped by user, handling edge cases like nulls and duplicates.
This question from PayPal's Phone Screen 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
- Write syntactically correct SQL with proper JOIN and WHERE clauses
- Use GROUP BY and aggregate functions appropriately
- Handle NULL values correctly in your queries
- Explain the query execution plan at a high level
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 would you do if this query needs to run every 5 minutes?
- How would you handle slowly changing dimensions in this scenario?
- 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 compute a cumulative sum of transactions grouped by user and date. We will be working with a dataset that typically includes columns like user_id, transaction_date, and amount. Th...
Approach
- Select Relevant Columns: Start by selecting
user_id,transaction_date, andamountfrom the transactions table. - Handle NULLs: Use
COALESCEto treat NULL amounts as zero. - **Aggr...
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