-- Клиенты, у которых есть хотя бы один заказ
SELECT c.full_name, c.email
FROM customers c
WHERE EXISTS (
    SELECT 1
    FROM orders o
    WHERE o.customer_id = c.customer_id
);

-- Клиенты без заказов
SELECT c.full_name
FROM customers c
WHERE NOT EXISTS (
    SELECT 1 FROM orders o WHERE o.customer_id = c.customer_id
);