What are the different types of mills?
select distinct type from mill
Which ones were built by American architects?
select distinct t1.type from mill as t1 join architect as t2 on t1.architect_id = t2.id where t2.nationality = 'American'
Include those built by Canadian architects as well.
select distinct t1.type from mill as t1 join architect as t2 on t1.architect_id = t2.id where t2.nationality = 'American' or t2.nationality = 'Canadian'