How many of those fault logs had faulty parts?
select count ( * ) 
from
 ( 
SELECT fault_log_entry_id FROM Fault_Log where fault_log_entry_id in  ( 
select fault_log_entry_id from Fault_Log_Parts ) 
   ) 
fault logs with faulty parts refers to the presence of fault_log_entry_id in Fault_Log_Parts; counting these logs refers to COUNT(*);