Question: From what date and to what date do the staff work on a project that has the most staff and has staff in a leader role?
SQL: 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'