"Show Me the Running Balance"

Concepts covered: sqlAggregateOver

The English of the question always sounds simple. 'For each transaction, show the account's running balance.' 'Show me revenue accumulated since the start of the year, broken down by day.' 'Compute the cumulative number of users who have ever logged in.' Each one is a running total. Each one wants the same SQL shape: a SUM with an OVER clause that tells the engine which rows to add up. What 'running' means, and how to spot it Why companies care Running totals power every financial and product-analytics dashboard. Account balance. Lifetime revenue. Cumulative active users. Year-to-date sales. The pattern is unavoidable. If you cannot write this query, an entire family of business questions is closed to you. That is why it is one of the most-asked SQL screen follow-ups.

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.

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.