Window function: lead/lag over user_id
Last updated: February 8, 2026
Quick Overview
Use window functions to compute rank partitioned by user_id.
Zscaler
Data Manipulation (SQL/Python)
Data Scientist
Zscaler
February 8, 2026Data Scientist
Technical Screen
Data Manipulation (SQL/Python)
Hard
440
5
1,975 solved
Use window functions to compute rank partitioned by user_id.
Zscaler 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
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 slowly changing dimensions in this scenario?
- 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?
- Can you rewrite this without using subqueries?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task involves using window functions to compute a rank for each user partitioned by user_id. The data likely consists of a table that includes at least user_id, a timestamp or a score colu...
Approach
- Identify the table containing user data, including
user_id, the metric for ranking (e.g.,scoreortimestamp), and any other relevant fields. - Use a Common Table Expression (CTE) to first...
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