What are SQL aggregate functions?

SELECT
  COUNT(*) AS total_rows,
  COUNT(email) AS emails_not_null,
  COUNT(DISTINCT country) AS unique_countries,
  SUM(amount) AS total_amount,
  AVG(amount) AS average_amount,
  MIN(amount) AS min_amount,
  MAX(amount) AS max_amount
FROM orders;

-- Count all rows
SELECT COUNT(*) FROM users;

-- Count non-NULL values
SELECT COUNT(phone) FROM users;

-- Count distinct values
SELECT COUNT(DISTINCT country) FROM users;

SELECT
  country,
  COUNT(*) AS user_count,
  AVG(age) AS avg_age
FROM users
GROUP BY country;

-- Multiple grouping columns
SELECT
  country,
  city,
  COUNT(*) AS user_count
FROM users
GROUP BY country, city;

-- WHERE filters rows, HAVING filters groups
SELECT
  category,
  COUNT(*) AS product_count,
  AVG(price) AS avg_price
FROM products
WHERE status = 'active'
GROUP BY category
HAVING COUNT(*) > 5 AND AVG(price) > 50
ORDER BY avg_price DESC;

SELECT
  COUNT(CASE WHEN status = 'active' THEN 1 END) AS active_count,
  COUNT(CASE WHEN status = 'inactive' THEN 1 END) AS inactive_count,
  SUM(CASE WHEN gender = 'M' THEN salary ELSE 0 END) AS male_salary_total
FROM employees;

-- MySQL
SELECT department, GROUP_CONCAT(name) AS employees
FROM employees GROUP BY department;

-- PostgreSQL
SELECT department, STRING_AGG(name, ', ') AS employees
FROM employees GROUP BY department;