Window function: lead/lag over category
Last updated: May 17, 2026
Quick Overview
Use window functions to compute rank partitioned by user_id.
Zoom
Data Manipulation (SQL/Python)
Data Scientist
Zoom
May 17, 2026Data Scientist
Phone Screen
Data Manipulation (SQL/Python)
Medium
184
6
4,446 solved
Use window functions to compute rank partitioned by user_id.
Zoom asks this during the Phone 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
- 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
Date/time manipulation
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
Pandas vectorized operations and groupby
Data cleaning and transformation
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
- How would you handle this if the data was spread across multiple databases?
- How would you optimize this query for a table with 100 million rows?
- How would you validate the correctness of your query results?
- What would you do if this query needs to run every 5 minutes?
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Practice SQL ProblemsSample Answer
Problem Understanding
In this problem, we need to analyze user activities in a dataset that includes information about users, their activities, categories, and timestamps. The goal is to compute a ranking of activities for...
Approach
To build the query, we will take the following steps:
- Identify the necessary fields: We need fields like
user_id,activity_id,category, andtimestampto compute the rank. - **Use a Co...
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