auto id with trigger

Hi i dont know why this trigger wont work all i want it to do is count how many entries are in the table then add one to create the primary key every time the table is updated

____________________________________________________CODE
____________________________________________________
CREATE OR REPLACE TRIGGER auto_id
BEFORE INSERT ON student
FOR EACH ROW
BEGIN
select count(ID) + 1 into :new.ID from dual;
END;
/

____________________________________________________
any ideas thanxs

I wouldn't use a trigger for this. Most DBMS has some built in mechanism for generating unique values. Oracle has sequences for instance.