Optimize a slow query on sessions
Last updated: July 29, 2025
Quick Overview
A query on sessions is running slowly. Identify the bottleneck and optimize it.
Bloomberg
Data Manipulation (SQL/Python)
Data Scientist
Bloomberg
July 29, 2025Data Scientist
Take-home Project
Data Manipulation (SQL/Python)
Easy
507
6
2,844 solved
A query on sessions is running slowly. Identify the bottleneck and optimize it.
This question from Bloomberg's Take-home Project 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
Date/time manipulation
Data cleaning and transformation
Aggregate functions and GROUP BY
NULL handling and COALESCE
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 this if the data was spread across multiple databases?
- How would you validate the correctness of your query results?
- What would you do if this query needs to run every 5 minutes?
- Can you rewrite this without using subqueries?
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Practice SQL ProblemsSample Answer
Problem Understanding
The data involved in this query pertains to user sessions, which typically includes fields such as session_id, user_id, start_time, end_time, and some performance metrics. The goal of the query is to ...
Approach
- Identify the Current Query: Review the existing SQL query to pinpoint areas that may cause performance issues, such as unnecessary JOINs, lack of indexes, or inefficient WHERE clauses.
- **Ana...
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