Automatically generate primary key value using backed Trigger in Oracle

PROBLEM : Automatically assign primary key value using backed Trigger. 

SOLUTION :  Code.
                   
CREATE OR REPLACE TRIGGER stlbas.dbt_auto_srlnum
BEFORE INSERT
   ON stlbas.stbrnlm1
   FOR EACH ROW
BEGIN
   BEGIN
      SELECT COUNT (a.sancid) + 1
        INTO :NEW.srlnum
        FROM stlbas.stbrnlm1 a
       WHERE a.sancid = :NEW.sancid;
   EXCEPTION
      WHEN NO_DATA_FOUND
      THEN
         :NEW.srlnum                := 1;
      WHEN OTHERS
      THEN
         :NEW.srlnum                := 1;
   END;
END;