Cumulative vs Grand Total

Concepts covered: sqlWindowFrame

Once the canonical running total works, the interviewer will pivot. 'Now do a 7-day rolling sum.' 'Now reset every month.' 'Now only count positive transactions.' These follow-ups test whether you understand the template or memorized the canonical query. Each variant changes exactly one piece of the OVER clause; the rest stays the same. 7-day rolling sum Change the frame clause. UNBOUNDED PRECEDING becomes 6 PRECEDING. The window now contains seven rows: the current row plus the six rows immediately before it. Everything else stays the same. ROWS BETWEEN 6 PRECEDING AND CURRENT ROW is seven rows, not six. The frame is inclusive on both sides. Count carefully: 6 PRECEDING through CURRENT ROW means six rows before plus the current row, which is seven rows total. Monthly reset Change the part

About This Interactive Section

This section is part of the Running Totals: Beginner 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.

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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.