CREATE TABLE customer_contact_channels (channel_code VARCHAR(255), contact_number VARCHAR(255), active_to_date VARCHAR(255), active_from_date VARCHAR(255))
What is the channel code and contact number of the customer contact channel that was active for the longest time?
    
SQL: SELECT channel_code, contact_number FROM customer_contact_channels WHERE active_to_date - active_from_date = (SELECT active_to_date - active_from_date FROM customer_contact_channels ORDER BY (active_to_date - active_from_date) DESC LIMIT 1)