oracle表设置主键自增长

CREATE OR REPLACE TRIGGER T_USERS/*触发器名称*/
  BEFORE INSERT ON T_USERS/*表名*/
  FOR EACH ROW
  WHEN (new.PK_USER /*自增长列名*/ is null)
begin
  select USERID.nextval/*自增长名*/ into :new.PK_USER from dual;
end;


create sequence USERID /*自增长名*/
minvalue 1
maxvalue 999999999999999999999999999
start with 1
increment by 1
cache 20;