Window function: running total over category
Last updated: September 18, 2025
Quick Overview
Use window functions to compute running total partitioned by date.
Grafana Labs
Data Manipulation (SQL/Python)
Data Scientist
Grafana Labs
September 18, 2025Data Scientist
Onsite
Data Manipulation (SQL/Python)
Hard
19
3
2,041 solved
Use window functions to compute running total partitioned by date.
Data manipulation questions at Grafana Labs test your ability to work with real-world datasets. This Onsite 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
Pandas vectorized operations and groupby
JOIN types and when to use each
NULL handling and COALESCE
Aggregate functions and GROUP BY
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 optimize this query for a table with 100 million rows?
- What indexes would you create to support this query?
- How would you validate the correctness of your query results?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task involves calculating a running total of a specified metric (e.g., sales, transactions) over a dataset that includes at least two key columns: 'date' and 'category'. The running total needs to...
Approach
- Identify the Dataset: Start by identifying the relevant table that contains the data. For instance, let's assume the table is named 'transactions' with columns 'date', 'category', and 'amount'....
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