What are window functions in SQL?

Window functions perform calculations across a set of rows related to the current row (a "window") without collapsing rows like GROUP BY. They are applied with the OVER() clause. Unlike aggregates (which produce one row per group), window functions return a value for every row while still being able to look at related rows. Key window functions: ROW_NUMBER() — assigns a unique sequential number to each row within the partition (1, 2, 3...); RANK() — like ROW_NUMBER but ties get the same rank and the next rank is skipped (1, 1, 3); DENSE_RANK() — ties get the same rank, next rank is not skipped (1, 1, 2); LAG(col, n) / LEAD(col, n) — access the value of a column n rows before/after the current row; SUM() OVER(), AVG() OVER() — running totals/averages; NTILE(n) — divides rows into n equal buckets; FIRST_VALUE() / LAST_VALUE(). OVER() syntax: ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC). MySQL 8.0+ supports window functions fully.