Window function: lead/lag over category
Last updated: January 6, 2026
Quick Overview
Use window functions to compute rank partitioned by user_id.
Mastercard
Data Manipulation (SQL/Python)
Data Scientist
Mastercard
January 6, 2026Data Scientist
Onsite
Data Manipulation (SQL/Python)
Easy
66
7
3,425 solved
Use window functions to compute rank partitioned by user_id.
Mastercard asks this during the Onsite 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
- Write syntactically correct SQL with proper JOIN and WHERE clauses
- Use GROUP BY and aggregate functions appropriately
- Handle NULL values correctly in your queries
- Explain the query execution plan at a high level
Key Topics to Cover
Pandas vectorized operations and groupby
Common Table Expressions (CTEs)
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
JOIN types and when to use each
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 slowly changing dimensions in this scenario?
- How would you validate the correctness of your query results?
- What indexes would you create to support this query?
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
In this problem, we need to analyze transaction data associated with users in order to compute a rank based on transaction amounts, partitioned by user_id. The dataset likely consists of columns suc...
Approach
- Identify the Source Table: Determine the main table containing transaction data.
- Select Relevant Columns: Focus on
user_id,transaction_date, andtransaction_amountfor ranking pur...
Submit Your Answer
Markdown supported