Show me the number of project staffs.
select count(*) from project_staff;
How many of those worked as leaders?
select count(*) from project_staff where role_code = 'leader';
How about the number of those who either worked as leaders or started working before '1989-04-24 23:51:54'?
select count(*) from project_staff where role_code = 'leader' or date_from < '1989-04-24 23:51:54';