Join sessions and sessions to find cumulative sum
Last updated: September 7, 2025
Quick Overview
Write a query joining sessions and messages to produce the combined cumulative sum.
DE Shaw
Data Manipulation (SQL/Python)
Data Scientist
DE Shaw
September 7, 2025Data Scientist
Technical Screen
Data Manipulation (SQL/Python)
Medium
13
6
502 solved
Write a query joining sessions and messages to produce the combined cumulative sum.
This question from DE Shaw's Technical Screen 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
- 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
Aggregate functions and GROUP BY
Index optimization and query performance
Data cleaning and transformation
NULL handling and COALESCE
Date/time manipulation
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 handle slowly changing dimensions in this scenario?
- What would you do if this query needs to run every 5 minutes?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task involves two primary tables: sessions and messages. The sessions table contains records of user sessions, likely with fields such as session_id, user_id, and `session_start_ti...
Approach
- Join the Tables: Start by joining the sessions table with the messages table on the
session_idfield. - Aggregate Message Counts: Use a
GROUP BYclause to count the number of m...
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