How many bridges did each architect build?
select count(*) from bridge group by architect_id
What are the ids, names and genders of the architects who built two bridges?
select t1.id, t1.name, t1.gender from architect as t1 join bridge as t2 on t1.id = t2.architect_id group by t1.id having count(*) = 2
How about for the architects who built two bridges or one mill?
select t1.id, t1.name, t1.gender from architect as t1 join bridge as t2 on t1.id = t2.architect_id group by t1.id having count(*) = 2 union select t1.id, t1.name, t1.gender from architect as t1 join mill as t2 on t1.id = t2.architect_id group by t1.id having count(*) = 1