Calculate cumulative sum per region
Last updated: November 8, 2025
Quick Overview
Write a query to compute cumulative sum grouped by date, handling edge cases like nulls and duplicates.
Zscaler
November 8, 202518
4
4,545 solved
Write a query to compute cumulative sum grouped by date, handling edge cases like nulls and duplicates.
This question from Zscaler'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
- Solve complex analytical problems with elegant, readable SQL
- Optimize queries for large-scale datasets with partitioning and indexing
- Use recursive CTEs, lateral joins, and advanced window functions
- Design the data model alongside the query solution
- Discuss trade-offs between SQL and programmatic approaches (Python/pandas)
- Consider the operational aspects: query scheduling, incremental processing
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?
- How would you validate the correctness of your query results?
- Can you rewrite this without using subqueries?
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Practice SQL ProblemsSample Answer
Problem Understanding
In this problem, we need to compute the cumulative sum of a certain metric (e.g., sales, revenue, etc.) grouped by date and further segmented by region. The data involved is likely stored in a table w...
Approach
- Identify the Source Table: Determine the table that contains the necessary data. Let's assume it is named
sales_data. - Initial Filtering: Filter out any rows where the metric is NULL si...
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