Window function: lead/lag over category
Last updated: January 17, 2026
Quick Overview
Use window functions to compute running total partitioned by date.
Meta
Data Manipulation (SQL/Python)
Data Scientist
Meta
January 17, 2026Data Scientist
Take-home Project
Data Manipulation (SQL/Python)
Hard
76
3
4,299 solved
Use window functions to compute running total partitioned by date.
Meta asks this during the Take-home Project 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)
JOIN types and when to use each
Date/time manipulation
Data cleaning and transformation
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?
- Can you rewrite this without using subqueries?
- How would you handle this if the data was spread across multiple databases?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task involves computing a running total of a specific metric, partitioned by date. The data includes timestamps and associated values that will be summed up. The output should reflect the cumulati...
Approach
- Identify Data Structure: Understand the structure of the table, which should include at minimum a 'date' column and a 'value' column (or similar).
- Use Window Functions: Utilize the `SU...
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