Explain what window functions are in database management systems and how they are used.

Window functions in database management systems are functions that perform calculations across a set of table rows that are related to the current row, without collapsing the result set into a single row like aggregate functions do.

They allow, for example, to compute moving sums, rank rows, calculate averages over a window of rows, etc., while preserving all the rows in the result.

An example of using a window function in SQL:

SELECT employee_id, department_id, salary,
       RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS salary_rank
FROM employees;

Here, the salary rank is calculated within each department for each employee, while all employees remain in the result.