What are all the service types?
select * from ref_service_types
How many times has each of them being performed?
select t1.service_type_description, t1.service_type_code, count(*) from ref_service_types as t1 join services as t2 on t1.service_type_code = t2.service_type_code group by t1.service_type_code
Show the description and code of the one that is performed the most often.
select t1.service_type_description, t1.service_type_code from ref_service_types as t1 join services as t2 on t1.service_type_code = t2.service_type_code group by t1.service_type_code order by count(*) desc limit 1