What are aggregate functions?

Aggregate functions in SQL are functions that perform calculations on a set of values and return a single final value. They are often used together with the GROUP BY operator to group data.

Main aggregate functions:

• COUNT() — counts the number of rows.
• SUM() — calculates the sum of values.
• AVG() — calculates the average value.
• MIN() — finds the minimum value.
• MAX() — finds the maximum value.

Example of usage:

SELECT department, COUNT(*) AS employee_count, AVG(salary) AS avg_salary
FROM employees
GROUP BY department;

This query counts the number of employees and the average salary for each department.