Counting after a JOIN.

A one-to-many join repeats the customer per order. GROUP BY c.id, c.name groups per customer even when names repeat. COUNT(o.id) gives zero for customers without orders; COUNT(*) counts the unmatched LEFT JOIN row too.

Small example:

SELECT c.id, COUNT(o.id) AS orders_count FROM customers c LEFT JOIN orders o ON c.id = o.customer_id GROUP BY c.id;