How many customers are there?
select count(*) from customers
Who have at least one accounts?
select t1.customer_first_name, t1.customer_last_name from customers as t1 join accounts as t2 on t1.customer_id = t2.customer_id
Who does not have any accounts?
select customer_first_name, customer_last_name from customers except select t1.customer_first_name, t1.customer_last_name from customers as t1 join accounts as t2 on t1.customer_id = t2.customer_id
Show his first names and last names.
select customer_first_name, customer_last_name from customers except select t1.customer_first_name, t1.customer_last_name from customers as t1 join accounts as t2 on t1.customer_id = t2.customer_id