DECLARE
   l_pin                        VARCHAR2(17);
   l_event_no                   VARCHAR2(9);
   l_causal_part_no             VARCHAR2(15);
   l_main_code                  VARCHAR2(8);
   l_event_id                   WC_MAIN_EVENT.event_id%TYPE;

BEGIN  
 l_pin := '5NPET46C67H271471';
 l_event_no := '058272000'; 
 l_causal_part_no := '883013K020';
 l_main_code := null;
 
  SELECT event_id INTO l_event_id 
  FROM WC_MAIN_EVENT
   WHERE event_group_id = 4
     AND pin = l_pin
     AND event_number = l_event_no
     AND part_number = l_causal_part_no 
     AND nvl2(l_main_code,event_code,1) = nvl2(l_main_code,l_main_code, 1); 
--     AND decode(l_main_code , NULL,1,event_code) = DECODE(l_main_code, NULL, 1, l_event_code);
  dbms_output.put_line('l_event_id = '|| l_event_id);

EXCEPTION
     when others then
   dbms_output.put_line('error = '|| sqlerrm);

END ;