oracle创建触发器，防止有人恶意删除表及常用对象。

CREATE OR REPLACE TRIGGER ddl_deny_lee

--jwz 20101018
    BEFORE  drop OR truncate on database
   
begin

  raise_application_error(num => -20000,
                          msg => '主机:' || SYS_CONTEXT('USERENV', 'HOST') ||
                                 chr(13) || 'IP:' ||
                                 SYS_CONTEXT('USERENV', 'IP_ADDRESS') ||
                                 '试图删除' || ora_dict_obj_name() || '表');
end;