List the details of the customers who do not have any policies.
SELECT customer_details FROM Customers EXCEPT SELECT T1.customer_details FROM Customers AS T1 JOIN Customer_Policies AS T2 ON T1.customer_id  =  T2.customer_id
customers who do not have any policies refers to selecting customers without matching records in Customer_Policies; list the details refers to selecting customer_details; EXCEPT is used to return the difference between two SELECT statements; JOIN is used to combine rows from two or more tables based on a related column between them.