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Notion·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Notion data engineer interview, SQL-focused technical screen. One question but they dug into it pretty thoroughly, including schema assumptions and how you'd handle edge cases.

Questions Asked (1)

Q1

Given an events table, write a SQL query to find the country with the highest total number of events. If there's a tie, return all tied countries.

Data ModelingTechnical Trade-offs
Author's notes

I went straight for GROUP BY country, COUNT(*), ORDER BY count DESC LIMIT 1, which works fine until they asked about ties.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the table schema and edge cases, then write a query that groups by country, counts events, and filters for the maximum count using a subquery or window function. Emphasize handling ties correctly and discuss performance considerations.

Pro tip: Mention that using a window function like RANK() or DENSE_RANK() is often more efficient and readable than a subquery, especially on large datasets, and it naturally handles ties.

1. Clarify requirements and schema

Ask about the table structure (e.g., columns: event_id, country, event_date) and whether 'country' can be NULL. Confirm that ties should return all countries.

2. Formulate the aggregation

Write a subquery or CTE that groups by country and counts the number of events per country.

3. Identify the maximum count

Determine the highest event count, either by using a subquery with MAX() or by ranking countries with a window function.

4. Filter for ties

Select all countries whose count equals the maximum count, ensuring ties are included.

5. Discuss performance and alternatives

Mention indexing on the country column, the trade-offs between subqueries and window functions, and how the query scales with data size.

Key Points to Mention

  • Use of GROUP BY and COUNT(*) for aggregation
  • Handling ties with either a subquery (WHERE count = (SELECT MAX(...))) or window function (RANK() OVER (ORDER BY count DESC) = 1)
  • Consideration of NULL values in the country column
  • Performance implications: indexing, avoiding full table scans, and using window functions for efficiency
  • Readability and maintainability of the query, especially for large datasets
  • Edge cases: empty table, all countries tied, or single country

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.