What are SQL aggregate functions? Provide examples.

Aggregate functions perform calculations on multiple rows and return a single value.

Common functions:
• COUNT() - counts rows
• SUM() - adds values
• AVG() - calculates average
• MAX() - finds maximum
• MIN() - finds minimum

Example:
SELECT COUNT(*) as total_users,
       AVG(age) as avg_age,
       MAX(salary) as max_salary
FROM employees;