How many tourists were there total?
select count ( * ) from visitors
How many tourist attractions?
select count ( distinct tourist_attraction_id ) from visits
How many tourists visited more than one tourist attraction?
select count ( * ) from ( select * from visits as t1 join visitors as t2 on t1.tourist_id = t2.tourist_id group by t1.tourist_id having count ( * ) >1 )
How many tourists didn't visit any tourist attractions?
select count ( * ) from visitors where tourist_id not in ( select tourist_id from visits )