How do COUNT, SUM, AVG, MIN, and MAX behave?

Aggregate functions operate on a set of rows and return one value. They are usually combined with GROUP BY, but without it the whole result set is one group.

SELECT
  COUNT(*) AS all_rows,
  COUNT(manager_id) AS rows_with_manager,
  COUNT(DISTINCT department_id) AS departments,
  AVG(salary) AS average_salary,
  MAX(salary) AS top_salary
FROM employees;

Important semantics

• COUNT(*) counts rows regardless of NULLs
• COUNT(column) and COUNT(DISTINCT column) exclude NULLs
• SUM and AVG ignore NULL inputs rather than treating them as zero
• AVG divides by the number of non-NULL values, not the total row count
• MIN and MAX return NULL when every input is NULL
• SUM over no rows returns NULL, while COUNT returns zero

If NULL should mean zero in a business calculation, use COALESCE explicitly. If the denominator matters, compute SUM and COUNT separately instead of relying on AVG.