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'
List from which date and to which date these staff work: project staff of the project which hires the most staffs
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 '
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 value ) union select date_from , date_to from project_staff where role_code = value
List from which date and to which date these staff work : project staff of the project which hires the most staffs