List from which date and to which date these staff work: project staff of the project which hires the most staffs
create table project_staff (date_from varchar, date_to varchar, project_id varchar, role_code varchar, PRIMARY KEY (date_from))
select date_from, date_to from project_staff where project_id in (select project_id from project_staff group by project_id order by count(*) desc limit 1) union select date_from, date_to from project_staff where role_code = 'leader'