Answer
Window functions (e.g., ROW_NUMBER(), RANK(), SUM() OVER (...)) allow calculations across sets of rows without collapsing them into a single result like GROUP BY does.
Why they are essential:
- Enable ranking, running totals, moving averages, percentiles, and gap/density analysis in a single pass.
- Reduce the need for self-joins and correlated subqueries, often resulting in cleaner and faster plans.
- Can be combined with PARTITION BY and ORDER BY to support rich analytical queries directly in OLTP or reporting databases.
- Help keep logic set-based and push computation into the database layer where it is highly optimized.