Join messages and messages to find rolling 7-day average
Last updated: December 2, 2025
Quick Overview
Write a query joining messages and sessions to produce the combined rolling 7-day average.
Notion
December 2, 202583
3
1,323 solved
Write a query joining messages and sessions to produce the combined rolling 7-day average.
This question from Notion's Phone 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
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 validate the correctness of your query results?
- What indexes would you create to support this query?
- 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 requires us to join two tables: messages and sessions. The goal is to compute a rolling 7-day average of some metric (e.g., number of messages sent) over the sessions. We need to ensure t...
Approach
To build the query, we will follow these steps:
- Common Table Expressions (CTEs): Create two CTEs for
messagesandsessionsto summarize the necessary counts by date. - JOIN: Use an INN...
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