create or replace trigger triname
before insert on table_name 
for each row
declare
  num number;
begin
  select seq_name.nextval into num from dual;
  :new.col := to_char(trunc(sysdate), 'yyyymmdd') ||'-'|| lpad(num, 5, '0');
end triname;

------解决方案--------------------

SQL> create table test (id varchar2(20),name varchar2(16));

表已创建。

SQL> create sequence seq_test_id
 2  start with 1
 3  increment by 1;

序列已创建。

SQL> create or replace trigger trig_test
 2    before insert on test
 3    for each row
 4  declare
 5    -- local variables here
 6  begin
 7      SELECT to_char(SYSDATE,'yyyymmdd')||'-'||lpad(seq_test_id.NEXTVAL,5,'0'
) INTO :new.id FROM dual;
 8  end trig_test;
 9  /

触发器已创建

SQL> insert into test (name) values ('AC米兰');

已创建 1 行。

SQL> commit;

提交完成。

SQL> select * from test;

ID                   NAME
-------------------- ----------------
20100324-00002       AC米兰

SQL>