How many project members were 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'
Kaç proje üyesi liderdi veya '1989-04-24 23:51:54' den önce çalışmaya başladı?