What are all the policy types of the customer that has the most policies listed?
SELECT DISTINCT t3.policy_type_code FROM customers AS t1 JOIN customers_policies AS t2 ON t1.customer_id   =   t2.customer_id JOIN available_policies AS t3 ON t2.policy_id   =   t3.policy_id WHERE t1.customer_name   =    ( SELECT t1.customer_name FROM customers AS t1 JOIN customers_policies AS t2 ON t1.customer_id   =   t2.customer_id GROUP BY t1.customer_name ORDER BY count ( * )  DESC LIMIT 1 ) 
customer with the most policies refers to MAX(COUNT(policy_id)); all the policy types of the customer refers to DISTINCT(policy_type_code);