Aggregate Functions — COUNT, SUM, AVG, MAX, MIN

SELECT COUNT(*) AS total_employees
FROM employees;

COUNT(*) counts all rows including NULLs. COUNT(column) counts non-NULL values in that column. Always alias your aggregates for readable output.

SELECT SUM(salary) AS total_salary
FROM employees;

SUM ignores NULL values. If you need to treat NULLs as 0 use SUM(COALESCE(salary, 0)).

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

When using aggregate functions with non-aggregate columns, every non-aggregate column must appear in GROUP BY.

SELECT department,
       MAX(salary) AS highest,
       MIN(salary) AS lowest
FROM employees
GROUP BY department;

You can combine multiple aggregate functions in one query. This is a common reporting pattern for salary band analysis.

SELECT department,
       AVG(salary) AS avg_salary
FROM employees
GROUP BY department
HAVING AVG(salary) > 50000;

HAVING filters after GROUP BY. You cannot use the alias avg_salary in the HAVING clause in most databases — repeat the expression.

SELECT department,
       COUNT(*) AS emp_count
FROM employees
GROUP BY department
HAVING COUNT(*) > 10
ORDER BY emp_count DESC;

This combines GROUP BY + HAVING + ORDER BY — a pattern asked frequently in e-commerce and banking sector SQL interviews in Noida/Gurgaon.