before，操作前的触发器

就是在操作进行以前处理的：
1.创建一个sequence：

SQL> create sequence mysequence
  2  start with 1
  3  increment by 1
  4  order nocycle;

2.表的结构：

 Name                       Null?    Type
 -------------------------- -------- -------------

 ID                         NOT NULL NUMBER
 NAME                                VARCHAR2(12)
 ADDRESS                             VARCHAR2(10)

3.触发器的创建语句：

 1  create or replace trigger incr_visitor
 2  before insert on visitor
 3  for each row
 4  declare
 5  sn number(10);
 6  begin
 7  select mysequence.nextval into sn from dual;
 8  :new.id:=sn;
 9* end;