What are PostgreSQL window functions and provide examples.

Window functions perform calculations across a set of table rows related to the current row.

Common Functions:
• ROW_NUMBER(): Sequential numbering
• RANK(): Ranking with gaps
• LAG()/LEAD(): Access previous/next rows
• SUM() OVER: Running totals

Example:

SELECT name, salary,
       ROW_NUMBER() OVER (ORDER BY salary DESC) as rank,
       SUM(salary) OVER (ORDER BY salary) as running_total
FROM employees;