Top-N Performance and Index Strategy

Concepts covered: sqlRankDenseRank

By this layer, the choice between ROW_NUMBER, RANK, and DENSE_RANK is not a syntax memory test. The interviewer is checking whether you can map the business question to the right function and articulate the failure modes of picking wrong. The function choice is a contract; the contract has consequences when ties exist in real data. The function-choice decision as a contract Each function answers a different question. ROW_NUMBER answers 'who are the N people we want to act on, exactly N, ties broken by policy.' RANK answers 'what are the top N positions, with ties sharing positions and consuming additional rows.' DENSE_RANK answers 'what are the top N distinct levels, with everyone at those levels included.' Naming the consumer's question is what determines the right function. The interview

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.