What are the names of all the buildings?
select name from buildings
Which ones have at least one company office?
select t2.name from office_locations as t1 join buildings as t2 on t1.building_id = t2.id join companies as t3 on t1.company_id = t3.id
How about at least two?
select t2.name from office_locations as t1 join buildings as t2 on t1.building_id = t2.id join companies as t3 on t1.company_id = t3.id group by t1.building_id having count(*) > 1