Window function: lead/lag over user_id
Last updated: August 30, 2025
Quick Overview
Use window functions to compute running total partitioned by user_id.
Netflix
Data Manipulation (SQL/Python)
Data Scientist
Netflix
August 30, 2025Data Scientist
Phone Screen
Data Manipulation (SQL/Python)
Easy
3
4
2,047 solved
Use window functions to compute running total partitioned by user_id.
This question from Netflix'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
- 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
Subqueries and correlated subqueries
Common Table Expressions (CTEs)
NULL handling and COALESCE
Index optimization and query performance
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
- What would you do if this query needs to run every 5 minutes?
- How would you handle slowly changing dimensions in this scenario?
- How would you optimize this query for a table with 100 million rows?
- Can you rewrite this without using subqueries?
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 task, we are dealing with a dataset that contains user activity records for Netflix. Each record includes columns such as user_id, activity_date, and activity_count. The goal is to compu...
Approach
To build the SQL query for this problem, we'll follow these steps:
- Select the required columns: We'll need
user_id,activity_date, and the cumulative sum ofactivity_count. - **Use a Win...
Submit Your Answer
Markdown supported