Aggregate functions

Aggregate functions (COUNT, SUM, AVG, MIN, MAX) turn many rows into a single value. Use GROUP BY to get one result per group and HAVING to filter the groups. WHERE filters rows before grouping; HAVING filters after.

SELECT COUNT(*) FROM orders;
SELECT AVG(total) FROM orders;

-- Orders per status
SELECT status, COUNT(*) AS count
FROM orders
GROUP BY status;

-- Categories with an average price over 20
SELECT category, AVG(price) AS avg_price
FROM products
WHERE in_stock = true
GROUP BY category
HAVING AVG(price) > 20;