You can use a subquery with NOT IN operator to find the customers who have not placed any orders as follows:
a) SELECT customer_id FROM customers WHERE customer_id NOT IN (SELECT customer_id FROM orders);
b) SELECT customer_id FROM orders WHERE customer_id NOT IN (SELECT customer_id FROM customers);
c) SELECT customer_id FROM customers WHERE customer_id NOT IN (SELECT order_id FROM orders);
d) SELECT customer_id FROM orders WHERE customer_id NOT IN (SELECT order_id FROM customers)
