Aggregate Functions

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Aggregate functions perform calculations on multiple rows of data and return a single result. Common aggregate functions:

Example:

				
					SELECT SUM(salary), AVG(salary), COUNT(*), MIN(salary), MAX(salary) 
FROM employees;

Use the COUNT function with a GROUP BY clause to count employees in each department.
Example:

				
					SELECT department_id, COUNT(*) AS employee_count 
FROM employees 
GROUP BY department_id;

The HAVING clause filters groups after applying aggregate functions.

Example (Find departments with more than 5 employees):

				
					SELECT department_id, COUNT(*) AS employee_count 
FROM employees 
GROUP BY department_id 
HAVING COUNT(*) > 5;

Key Note: Use WHERE for row-level filtering and HAVING for filtering aggregated data.

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