List all the available service names.
select service_name from services
Which of these have been used in some party service?
select t1.service_name from services as t1 join party_services as t2 on t1.service_id = t2.service_id
How about those never used in any party service?
select service_name from services except select t1.service_name from services as t1 join party_services as t2 on t1.service_id = t2.service_id