Optimize a slow query on transactions
Last updated: July 28, 2025
Quick Overview
A query on transactions runs slowly. Identify the bottleneck and optimize.
Walmart
Data Manipulation (SQL/Python)
Data Scientist
Walmart
July 28, 2025Data Scientist
Technical Screen
Data Manipulation (SQL/Python)
Medium
42
9
236 solved
A query on transactions runs slowly. Identify the bottleneck and optimize.
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.
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Practice SQL ProblemsSample Answer
Problem Understanding
The dataset involved is a transactions table that logs individual transaction details at Walmart. The query needs to produce insights on transaction counts, total sales, and possibly additional metric...
Approach
- Examine the Current Query: Start by reviewing the existing SQL query to identify any inefficient joins, lack of indexing, or unnecessary calculations.
- Identify Key Filters: Determine wh...
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