Now that you know how to use aggregate functions like SUM(), AVG(), etc., let’s talk about how to use them per category, per product, or per customer using *GROUP BY*. *Why GROUP BY?* Use GROUP BY when you want to aggregate data separately for each value in a column. *Example: Total Sales per Product* SELECT product, SUM(quantity_sold) AS total_quantity FROM sales GROUP BY product; This query shows how many units of each product were sold. Without GROUP BY, you'd only get one total for all products combined. *Another Example: Revenue per Region* SELECT region, SUM(price * quantity_sold) AS revenue FROM sales GROUP BY region; Useful for business insights like which region brings the most revenue. *Golden Rule:* - Every column in the SELECT clause (that’s not inside an aggregate function) must be in the GROUP BY. So this is valid: 👇 SELECT customer_id, COUNT(*) FROM orders GROUP BY customer_id; But this will throw an error: 👇 SELECT customer_id, COUNT(*), status FROM orders GROUP BY customer_id; -- ❌ 'status' is not aggregated or grouped *React with ❤️ if you're ready for the next quiz.* 📝 SQL Learning Series: https://whatsapp.com/channel/0029VanC5rODzgT6TiTGoa1v/1075