What is the number of tourists who didn't make any visit?
select count ( * ) from visitors where tourist_id not in ( select tourist_id from visits )
What are their details?
select tourist_details from visitors where tourist_id not in ( select tourist_id from visits )
What are the details of the tourist who had the earliest visit?
select t1.tourist_details from visitors as t1 join visits as t2 on t1.tourist_id = t2.tourist_id order by visit_date limit 1