Show me the project ids of the projects which did not hire any staff for a leader role.
select project_id from projects where project_id not in ( select project_id from project_staff where role_code = 'leader' )
How about those without a researcher role?
select project_id from projects where project_id not in ( select project_id from project_staff where role_code = 'researcher' )
What are their details?
select project_details from projects where project_id not in ( select project_id from project_staff where role_code = 'researcher' )