How many mills did each architect build?
select count(*) from mill group by architect_id
Which architect built the most?
select * from architect as t1 join mill as t2 on t1.id = t2.architect_id group by t1.id order by count(*) desc limit 1
What are the id, name and nationality of this architect?
select t1.id, t1.name, t1.nationality from architect as t1 join mill as t2 on t1.id = t2.architect_id group by t1.id order by count(*) desc limit 1