Write a query to find percentile rank from events

Last updated: November 3, 2025

Quick Overview

Write a SQL query to calculate percentile rank from the events table, considering nulls and duplicates.

Discord

Data Manipulation (SQL/Python)

Data Scientist

Discord

November 3, 2025

Data Scientist

Phone Screen

Data Manipulation (SQL/Python)

Medium

1,689 solved

Write a SQL query to calculate percentile rank from the events table, considering nulls and duplicates.

Data manipulation questions at Discord 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

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

Common Table Expressions (CTEs)

Date/time manipulation

Window functions (ROW_NUMBER, RANK, LAG, LEAD)

JOIN types and when to use each

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?
How would you handle slowly changing dimensions in this scenario?

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Practice SQL Problems

Sample Answer

Problem Understanding

The task involves calculating the percentile rank of events from a table named events. The events table consists of several columns, including, but not limited to, event_id, user_id, `event_va...

Approach

Select Relevant Columns: Identify the columns needed for the calculation, primarily event_value for ranking.
Handle Nulls: Use a CTE to filter out NULL event_values, as they should ...

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