1,创建sequence:
　　create sequence SEQ_SM_USER

　　minvalue 1

　　maxvalue 999999999999999999999999999

　　start with 1

　　increment by 1

　　cache 20;

　　2,创建触发器:

　　create or replace trigger tg_sm_user

　　before insert on sm_user

　　referencing old as oldval

　　new as newval

　　for each row

　　begin

　　select seq_sm_user.nextval into :newval.pk_user from dual;

　　end;