What is a window function?

A window function performs calculations across related rows without collapsing them.

Example:

SELECT
    employee,
    department,
    salary,
    AVG(salary) OVER (PARTITION BY department) AS dept_avg
FROM employees;

Every employee remains a row.

Compare:

GROUP BY department

which collapses multiple employees into one department row.

Difference:

> GROUP BY reduces rows. > Window functions keep rows.