Hi,
Below is the query am using..
create or replace trigger tr_stud
after insert or update or delete
of stud_code , name
on stud
referencing old as old new as new
for each row
begin
update lib set current_date =current_date
where
(select c.lib_id from cst c, stud t
where t.stud_code=c.stud_id);
end;