Window function: running total over category
Last updated: August 25, 2025
Quick Overview
Use window functions to compute running total partitioned by user_id.
Two Sigma
Data Manipulation (SQL/Python)
Data Scientist
Two Sigma
August 25, 2025Data Scientist
Technical Screen
Data Manipulation (SQL/Python)
Medium
230
3
2,807 solved
Use window functions to compute running total partitioned by user_id.
This question from Two Sigma's Technical 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
- 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
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
NULL handling and COALESCE
JOIN types and when to use each
Subqueries and correlated subqueries
Pandas vectorized operations and groupby
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?
- Can you rewrite this without using subqueries?
- How would you handle slowly changing dimensions in this scenario?
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Practice SQL ProblemsSample Answer
Problem Understanding
The goal is to compute a running total for each user, partitioned by user_id. The relevant data involves a table (let's call it transactions) that includes at least the following columns: `user_id...
Approach
- Start by selecting the necessary columns from the
transactionstable:user_id,transaction_date, andamount. - Use the
ORDER BYclause to sort the transactions bytransaction_datewith...
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