请问:关于建立触发器出错的问题?

[ml74]

当我建立一个如下触发器时:
creat trigger tr_1
before insert on table_name_1
begin
select id_1.nextval into :new.column_name_1 from dual;
end;
(注:我建立此触发器的目的在于自动实现某表的某列可以自动递增,
id_1为我已经建好的sequence)

出现编译错误:
ORA-04082:NEW或OLD引用不允许在表层触发器中

因此,就不能建立触发器,请问我该如何解决?

[beckhambobo]

creat trigger tr_1
before insert on table_name_1
for each row
begin
select id_1.nextval into :new.column_name_1 from dual;
end;