Window function: rank over category
Last updated: December 21, 2025
Quick Overview
Use window functions to compute running total partitioned by date.
Plaid
Data Manipulation (SQL/Python)
Data Scientist
Plaid
December 21, 2025Data Scientist
Technical Screen
Data Manipulation (SQL/Python)
Hard
38
8
4,061 solved
Use window functions to compute running total partitioned by date.
Plaid asks this during the Technical Screen because data engineering skills are critical for the role. You should be comfortable with complex joins, window functions, CTEs, and performance optimization.
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
NULL handling and COALESCE
Common Table Expressions (CTEs)
Aggregate functions and GROUP BY
Index optimization and query performance
Subqueries and correlated subqueries
JOIN types and when to use each
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 validate the correctness of your query results?
- 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 (e.g., revenue, sales) partitioned by date, using window functions in SQL. The data involved will be a table containing at least the followi...
Approach
- Identify the Source Table: Determine the main table that contains the relevant data (e.g.,
transactions). - Basic SQL Structure: Start with a SELECT statement to retrieve the necessary...
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