Exactly-N vs At-Least-N Semantics

Concepts covered: sqlRowNumber

The closing escalation is about scale. The interviewer wants to know whether you can support this query at production volume. The right answer covers three layers: the supporting index, the partition or cluster layout, and the materialization strategy when the query powers a dashboard. Hitting all three is what flips this question from a basic SQL screen to a verdict the interviewer remembers at the debrief. The supporting index ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) requires the engine to sort each department's rows by salary. An index on (department_id, salary DESC) lets the engine walk the index in partition + order without a sort step. Without that index, the engine has to scan the entire employees table and sort it in memory per partition (or spill to disk

About This Interactive Section

This section is part of the Top N Per Group: Intermediate lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

How DataDriven Lessons Work

DataDriven combines four interview rounds (SQL, Python, Data Modeling, Pipeline Architecture) with adaptive difficulty and spaced repetition. Easy problems get harder as you improve. Weak concepts resurface until you master them. Your readiness score tracks progress across every topic interviewers test. Every lesson section ends with problems you solve by writing and running real code, not by picking multiple-choice answers.