trigger for RIB suppliers

Hi Team,

I want to create a trigger PL/SQL for the table: ap_supplier_sites_all  (oracle EBS R12), in this table i have below information:  vendor_site_id (primary key), vendor_id, attribute4 (varchar250), my trigger will be developp for avoid the insert or update the attribute4 if this attribute4 exists in the table for each vendor_id,

i do two triggers:

1first trigger:

CREATE OR REPLACE TRIGGER TRIG

BEFORE INSERT OR UPDATE of attribute4 ON ap_supplier_sites_all

referencing new as new old as old

for each row

BEGIN

         IF :new.attribute4 in (123,789,78965,12345)  -- (select distinct attribute4 from ap_supplier_sites_all) --(123,789,78965,12345)  trigger and vendor_id not in