mysql中如何实现字段的添加、修改、删除_MySQL:alter table tabelname add new_field_id int(5) unsigned default 0 not null auto_increment ,add primary key (new_field_id);//增加一个新列 alter table t2 add d timestamp;alter table infos add ex tinyint not null default '0';//删除列 alter table t2 drop