Window function: lead/lag over user_id
Last updated: March 5, 2026
Quick Overview
Use window functions to compute rank partitioned by date.
Elastic
Data Manipulation (SQL/Python)
Data Scientist
Elastic
March 5, 2026Data Scientist
Phone Screen
Data Manipulation (SQL/Python)
Medium
129
7
3,798 solved
Use window functions to compute rank partitioned by date.
This question from Elastic'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
- 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
JOIN types and when to use each
Common Table Expressions (CTEs)
NULL handling and COALESCE
Window functions (ROW_NUMBER, RANK, LAG, LEAD)
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?
- Can you rewrite this without using subqueries?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task is to compute a rank for users based on their activity, partitioned by date. The relevant data likely includes user IDs, activity timestamps, and possibly other metrics that indicate user eng...
Approach
- Identify the Dataset: Ensure we have a table that contains user activity data with at least the columns
user_id,activity_date, andactivity_metric(or similar). - Use a CTE: Creat...
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