Window function: rank over date
Last updated: April 6, 2026
Quick Overview
Use window functions to compute running total partitioned by user_id.
Notion
Data Manipulation (SQL/Python)
Data Scientist
Notion
April 6, 2026Data Scientist
Phone Screen
Data Manipulation (SQL/Python)
Hard
50
13
3,771 solved
Use window functions to compute running total partitioned by user_id.
Data manipulation questions at Notion test your ability to work with real-world datasets. This Phone Screen question evaluates your SQL proficiency, understanding of data modeling, and ability to derive insights from raw data.
What the Interviewer Expects
- Solve complex analytical problems with elegant, readable SQL
- Optimize queries for large-scale datasets with partitioning and indexing
- Use recursive CTEs, lateral joins, and advanced window functions
- Design the data model alongside the query solution
- Discuss trade-offs between SQL and programmatic approaches (Python/pandas)
- Consider the operational aspects: query scheduling, incremental processing
Key Topics to Cover
Date/time manipulation
Subqueries and correlated subqueries
JOIN types and when to use each
Data cleaning and transformation
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?
- How would you optimize this query for a table with 100 million rows?
- Can you rewrite this without using subqueries?
- What would you do if this query needs to run every 5 minutes?
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Practice SQL ProblemsSample Answer
Problem Understanding
The problem involves a dataset containing user activity logs, where each entry has a user_id, a date, and possibly other columns (like activity or amount). The goal is to compute a running tot...
Approach
- Identify the required fields: We need
user_id,date, and the numeric field to sum (e.g.,amount). - Determine the aggregation: We will use a window function with
SUM()to calculate...
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