SQL Window Functions Explained: Stop Collapsing Your Data with GROUP BY

You're writing a query to find each employee's salary alongside the average salary for their department. Your first instinct might be to reach for GROUP BY — but then you'd lose the individual employee rows. So you write a subquery, or a join against an aggregated CTE, and suddenly a simple question becomes a 20-line monster. Window functions exist precisely to solve this. They let you perform calculations across a set of related rows without collapsing your result set. The row stays intact; the