Table schema:
CREATE TABLE Project_Staff (date_from VARCHAR, date_to VARCHAR, project_id VARCHAR, role_code VARCHAR)

Question: List from which date and to which date these staff work: project staff of the project which hires the most staffs
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'