Yes
select customer_details from customers
Show the ID of the customers without policies
select customer_id from customers except select t2.customer_details from policies as t1 join customers as t2 on t1.customer_id = t2.customer_id
What are the customer details with those customers
select customer_details from customers except select t2.customer_details from policies as t1 join customers as t2 on t1.customer_id = t2.customer_id