Using trigger to audit delete on a table

create or replace trigger a_trigger
before delete on a
declare
username varchar2(20);
ipaddress varchar2(20);
sessionuser varchar2(20);
begin
SELECT sys_context(‘USERENV’,’OS_USER’) into username FROM dual;
SELECT sys_context(‘USERENV’,’IP_ADDRESS’) into ipaddress FROM dual;
SELECT sys_context(‘USERENV’,’SESSION_USER’) into sessionuser from dual;
insert a_delete_history (trigger_time,os_username,sess_username,ip_address) values (sysdate,username,sessionuser,ipaddress);
end a_trigger;