SUM OVER (ORDER BY ...) Basics

Concepts covered: sqlWindowFrame

The canonical query has the same shape every time. SUM of the column to total, OVER with three parts: PARTITION BY for the group, ORDER BY for the time column, and a frame clause that says which rows to include. Memorize this shape. Type it without thinking. The query you should be able to write from memory Read the query out loud Each clause does a specific job. SUM(amount) is the value you are accumulating. PARTITION BY account_id makes each account its own running total; the balance restarts at every new account. ORDER BY txn_date defines the order of accumulation; rows are added in date order. ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW is the frame; it says 'sum every row from the first row in this partition up to and including this row.' Together, those four pieces produce a per

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.