Lesson 5 of 7
GROUP BY: aggregate by group
Aggregate functions (COUNT, SUM, AVG…) combine with GROUP BY to compute per group: orders per customer, total per city…
SELECT customer_id, COUNT(*), SUM(amount) FROM orders GROUP BY customer_id — one result row per customer.
SELECT city, COUNT(*) FROM customers GROUP BY city;
🎯 Exercise
On the orders table: return, per customer_id, the number of orders (COUNT(*)) and the sum of amounts (SUM(amount)). Group by customer_id and sort by customer_id.
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