Window function: lead/lag over user_id
Last updated: May 2, 2026
Quick Overview
Use window functions to compute rank partitioned by user_id.
Microsoft
Data Manipulation (SQL/Python)
Data Scientist
Microsoft
May 2, 2026Data Scientist
Technical Screen
Data Manipulation (SQL/Python)
Medium
96
15
2,282 solved
Use window functions to compute rank partitioned by user_id.
Microsoft 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
- 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
Data cleaning and transformation
NULL handling and COALESCE
Date/time manipulation
JOIN types and when to use each
Pandas vectorized operations and groupby
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?
- What would you do if this query needs to run every 5 minutes?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task is to compute the rank of records partitioned by user_id using SQL window functions. We must assume there is a dataset containing user interactions, which may include columns like user_id...
Approach
- Select Data: Start by selecting the necessary columns including
user_id,interaction_time, and any other relevant fields. - Order Data: Use the
ORDER BYclause within the window fun...
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