What are aggregate functions in SQL? Give examples.

Aggregate functions perform calculations on sets of rows and return a single value. Common functions: COUNT (number of rows), SUM (total of numeric column), AVG (average value), MIN (smallest value), MAX (largest value). Used with GROUP BY to calculate aggregates per group. Example: SELECT department, AVG(salary) FROM employees GROUP BY department. COUNT(*) counts all rows; COUNT(column) counts non-null values.