Rolling Windows (7-Day Trailing)

Concepts covered: sqlWindowFrame

This is the section the interviewer goes deepest on. The frame clause is where they probe whether you understand the silent bug that ships running totals to production with wrong numbers. The bug does not throw an error. It does not warn. It produces visibly plausible numbers that are subtly wrong. Anyone who has been on call for a reporting pipeline has seen this bug. Anyone who has not is about to learn it the hard way. The bug, stated plainly If two rows share the same ORDER BY value, RANGE treats them as a single peer group and assigns them the same cumulative value. ROWS treats them as individual rows and assigns each a distinct cumulative value. Most candidates have never seen this difference because their toy examples use unique dates. Real transaction data has many rows per day. Th

About This Interactive Section

This section is part of the Running Totals: 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.