Window function: running total over date
Last updated: August 18, 2025
Quick Overview
Use window functions to compute running total partitioned by user_id.
Reddit
Data Manipulation (SQL/Python)
Data Scientist
Data Scientist
Technical Screen
Data Manipulation (SQL/Python)
Medium
27
6
2,334 solved
Use window functions to compute running total partitioned by user_id.
This question from Reddit'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
Subqueries and correlated subqueries
Date/time manipulation
Data cleaning and transformation
Index optimization and query performance
Pandas vectorized operations and groupby
NULL handling and COALESCE
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
- Can you rewrite this without using subqueries?
- What indexes would you create to support this query?
- How would you optimize this query for a table with 100 million rows?
Sharpen Your Skills on Codemia
Practice similar problems with our interactive workspace, get AI feedback, and track your progress.
Practice SQL ProblemsSample Answer
Problem Understanding
The task is to compute a running total of a specific metric (e.g., post scores, comment counts) for each user on Reddit, partitioned by user_id, and ordered by date. The data involved includes a tab...
Approach
- Identify the Table: Begin with the
user_activitytable which contains the necessary data. - Use Window Functions: Employ the
SUM()window function to calculate the running total.
3...
Submit Your Answer
Markdown supported