"Now Do It Without Window Functions"

Concepts covered: sqlRowNumber

The question opens with 'top 3 highest-paid employees per department.' By the third follow-up, the conversation is about how to maintain that view continuously as the employees table changes, what happens when a region's headcount triples, and whether the right answer is a materialized view, a precomputed table, or an on-demand query. The SQL itself is settled in the first two minutes. The remaining time is about whether you can design the system that owns the SQL. What the interviewer is actually testing at this level Why this question stays in staff loops Top N per group is a microcosm of every reporting design question a staff engineer faces. The query has business semantics (tie handling), physical considerations (indexes and partitions), and operational concerns (refresh cadence, down

About This Interactive Section

This section is part of the Top N Per Group: Advanced 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.