Window function: lead/lag over category
Last updated: December 1, 2025
Quick Overview
Use window functions to compute running total partitioned by date.
Snowflake
Data Manipulation (SQL/Python)
Data Scientist
Snowflake
December 1, 2025Data Scientist
Take-home Project
Data Manipulation (SQL/Python)
Medium
567
6
4,360 solved
Use window functions to compute running total partitioned by date.
Snowflake 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
- 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
Subqueries and correlated subqueries
Common Table Expressions (CTEs)
JOIN types and when to use each
Aggregate functions and GROUP BY
Index optimization and query performance
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
- What indexes would you create to support this query?
- Can you rewrite this without using subqueries?
- How would you handle this if the data was spread across multiple databases?
- How would you validate the correctness of your query results?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task requires us to compute a running total, partitioned by date, using window functions in SQL. The data involved likely consists of sales or transaction records with at least the following colum...
Approach
- Identify the Source Data: Start by selecting the relevant columns from the transactions table, possibly named
transactions, which includesdate,category, andamount. - **Create CTE fo...
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