Context

Running totals are a common SQL pattern in reporting, finance, and product analytics. Interviewers ask this to test whether you understand cumulative calculations and the SQL features used to produce them efficiently.

Core question

Explain how you would calculate a running total in SQL. In your answer, describe:

1. What a running total is
2. Which SQL feature is typically used to calculate it
3. Why ORDER BY is required in the calculation
4. How partitioning changes the result
5. What common mistakes can lead to incorrect totals

Scope guidance

The interviewer is usually looking for a practical explanation centered on window functions, especially SUM() OVER (...). You should be able to explain the difference between a regular aggregate and a windowed aggregate, and give a simple example such as cumulative sales by date or cumulative revenue by customer.