ALTER TABLE 表名 MODIFY 列名 数据类型; 

> create table t1 (id int,
    name varchar(5),
    rmb decimal(9,1));

alter table t1 modify name varchar(7);

> use test;
> desc t1;
+-------+--------------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id  | int(11)   | YES |   | NULL  |    |
| name | varchar(5)  | YES |   | NULL  |    |
| rmb  | decimal(9,1) | YES |   | NULL  |    |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

> create table t1_tmp (id int,
  name varchar(10),
  rmb decimal(9,1));

> flush tables with read lock;  先锁住表，放在表被打开，以免数据丢失。
 > system cp /usr/local/mariadb/var/test/t1_tmp.frm /usr/local/mariadb/var/test/t1.frm

> unlock tables;

> show create table t1\G
*************************** 1. row ***************************
    Table: t1
Create Table: CREATE TABLE `t1` (
 `id` int(11) DEFAULT NULL,
 `name` varchar(10) DEFAULT NULL,
 `rmb` decimal(9,1) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

> insert into t1 values(2,'hechuangyang',3.8); 不报错的话就是修改成功了。