Count exhibitions for each artist.
select count(*) from exhibition as t1 join artist as t2 on t1.artist_id = t2.artist_id group by t1.artist_id
Which artist has the most exhibitions?
select * from exhibition as t1 join artist as t2 on t1.artist_id = t2.artist_id group by t1.artist_id order by count(*) desc limit 1
Give me his or her name and country.
select t2.name, t2.country from exhibition as t1 join artist as t2 on t1.artist_id = t2.artist_id group by t1.artist_id order by count(*) desc limit 1