Calculate percentile rank per date
Last updated: January 8, 2026
Quick Overview
Write a query to compute percentile rank grouped by date, handling edge cases like nulls and duplicates.
Snowflake
January 8, 20261
4
2,351 solved
Write a query to compute percentile rank grouped by date, handling edge cases like nulls and duplicates.
Snowflake asks this during the Onsite because data engineering skills are critical for the role. You should be comfortable with complex joins, window functions, CTEs, and performance optimization.
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
- What indexes would you create to support this query?
- How would you handle this if the data was spread across multiple databases?
- 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 is to calculate the percentile rank of values per date. We need to work with a dataset that includes at least two columns: a 'date' column and a 'value' column. The output should provide each...
Approach
- Data Cleaning: Start by filtering out any NULL values in the 'value' column since they cannot be ranked.
- CTE for Percentile Calculation: Use a Common Table Expression (CTE) to calculate ...
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