List the names of all the customers.
select customer_name from customers
What are the names of customers who have made some orders before?
select t1.customer_name from customers as t1 join customer_orders as t2 on t1.customer_id = t2.customer_id
Which customers never made an order?
select customer_name from customers except select t1.customer_name from customers as t1 join customer_orders as t2 on t1.customer_id = t2.customer_id