What is a window function in SQL and a typical use case?

Answer

A window function computes a value over a “window” of rows related to the current row without collapsing rows like GROUP BY. Use cases: ranking (ROW_NUMBER), running totals, “top N per group”.

SELECT user_id,
       amount,
       ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY created_at DESC) AS rn
FROM payments;