Window function: lead/lag over category
Last updated: July 22, 2025
Quick Overview
Use window functions to compute running total partitioned by date.
Square/Block
Data Manipulation (SQL/Python)
Data Scientist
Square/Block
July 22, 2025Data Scientist
Onsite
Data Manipulation (SQL/Python)
Easy
531
3
1,659 solved
Use window functions to compute running total partitioned by date.
Square/Block asks this during the Onsite 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
- 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
Pandas vectorized operations and groupby
Data cleaning and transformation
Subqueries and correlated subqueries
NULL handling and COALESCE
Aggregate functions and GROUP BY
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?
- How would you optimize this query for a table with 100 million rows?
- How would you handle slowly changing dimensions in this scenario?
- How would you validate the correctness of your query results?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task involves computing a running total of some numeric value (let's assume it's 'amount') partitioned by date. The data will likely come from a table that contains at least the following columns:...
Approach
To solve this problem, we will follow these steps:
- Select Relevant Columns: Identify and select the necessary columns from the source table.
- Handle NULL Values: Use COALESCE to manage an...
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