How to declare one variable for each row in trigger? - oracle

Question

I have following trigger:
create or replace TRIGGER MY_TIGGER_NAME AFTER UPDATE ON MY_TABLE
REFERENCING OLD AS OLD NEW AS NEW FOR EACH ROW
WHEN ( NEW.STATUS = ANY (10,40,42,44,46,50,60) and OLD.STATUS != NEW.STATUS)
DECLARE
PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
IF :NEW.ALERT is NULL
THEN
dbms_alert.signal('print_update_event','update_message');
ELSE
dbms_alert.signal( :NEW.ALERT,'update_message');
END IF;
commit;
END;
I would like to change it because it send alert for each row. I would like to send only one alert if more than one row with ALERT column equal NULL was updated and I would like to send one alert for each row with ALERT column NOT equal NULL.
As I understand Oracle variables I can declare local variable in my trigger but this variable will be declared separately for each row so following change has no sense:
create or replace TRIGGER MY_TIGGER_NAME AFTER UPDATE ON MY_TABLE
REFERENCING OLD AS OLD NEW AS NEW FOR EACH ROW
WHEN ( NEW.STATUS = ANY (10,40,42,44,46,50,60) and OLD.STATUS != NEW.STATUS)
DECLARE
PRAGMA AUTONOMOUS_TRANSACTION;
flag NUMBER(1,0) :=0;
BEGIN
IF :NEW.ALERT is NULL and flag=0
THEN
dbms_alert.signal('print_update_event','update_message');
flag:=1;
ELSE
dbms_alert.signal( :NEW.ALERT,'update_message');
END IF;
commit;
END;
Package variable could be used instead local variable but I think that package variable is bad idea because two triggers can be executed in parallel. Maybe I am wrong.
I attach diagram of trigger which shows what I would like to achieve.
How to do it?