Given the following relations: Artist (ID, name, genre) Tour (name, year, artistID) 'artistID' is a foreign key referring to 'ID' in 'Artist'. This value cannot be NULL. Show (venue, tour name, city) 'tour name' is a foreign key referring to 'name' in 'Tour'. This value cannot be NULL. Customer (ID, name, date of birth) Ticket (customerID, venue, tour name) 'customerID' is a foreign key referring to 'ID' in 'Customer'. This value cannot be NULL. 'venue' is a foreign key referring to 'venue' in 'Show'. This value cannot be NULL. 'tour name' is a foreign key referring to 'name' in 'Tour'. This value cannot be NULL. Which statement is correct?