Window function: rank over user_id
Last updated: August 8, 2025
Quick Overview
Use window functions to compute running total partitioned by user_id.
Square/Block
Data Manipulation (SQL/Python)
Data Scientist
Square/Block
August 8, 2025Data Scientist
Phone Screen
Data Manipulation (SQL/Python)
Hard
5
7
1,450 solved
Use window functions to compute running total partitioned by user_id.
This question from Square/Block'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
JOIN types and when to use each
Data cleaning and transformation
NULL handling and COALESCE
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
Aggregate functions and GROUP BY
Date/time manipulation
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
- How would you handle this if the data was spread across multiple databases?
- What would you do if this query needs to run every 5 minutes?
- How would you handle slowly changing dimensions in this scenario?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task is to compute a running total of a specific metric, partitioned by user_id. This means that for each user, we need to calculate a cumulative sum that resets whenever we encounter a new `use...
Approach
- Identify the Relevant Columns: We need
user_id,transaction_amount, and possiblytransaction_datefor ordering. - Use Window Functions: We will utilize the
SUM()window function to...
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