Aggregate Functions

COUNT() SUM() AVG() MIN() MAX()

SELECT COUNT(*) AS employee_count FROM employees;
SELECT SUM(salary) AS total_salary FROM employees;
SELECT AVG(salary) AS average_salary FROM employees;
SELECT MIN(salary), MAX(salary) FROM employees;

GROUP BY

GROUP BY groups rows that share the same values and lets you calculate aggregates for each group.

SELECT department_id, AVG(salary) AS average_salary
FROM employees
GROUP BY department_id;

HAVING

HAVING filters groups after aggregation, unlike WHERE which filters individual rows before grouping.

SELECT department_id, AVG(salary) AS average_salary
FROM employees
GROUP BY department_id
HAVING AVG(salary) > 75000;

Filters groups, not rows.

WHERE vs HAVING

Understanding WHERE vs HAVING is crucial: WHERE filters rows before grouping, HAVING filters groups after aggregation.

Rule: WHERE → individual rows, HAVING → grouped results