21.1 命令语法及默认添加演示
1、命令语法：alter table <表名> add <字段> <类型> 其他
2、测试表数据：
mysql> use oldboy;
Database changed
mysql> show create table test\G;
*************************** 1. row ***************************
Table: test
Create Table: CREATE TABLE `test` (
`id` int(4) NOT NULL DEFAULT '0',
`age` tinyint(2) DEFAULT NULL,
`name` varchar(16) DEFAULT NULL,
`shouji` char(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `index_name` (`name`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8
1 row in set (0.09 sec)
mysql> desc test;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id | int(4) | NO | PRI | 0 | |
| age | tinyint(2) | YES | | NULL | |
| name | varchar(16) | YES | MUL | NULL | |
| shouji | char(11) | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+
4 rows in set (0.13 sec)
3、实践案例
例如：在表 test 中添加字段 sex
4、执行的命令演示
添加性别列，默认语句
mysql> desc test;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id | int(4) | NO | PRI | 0 | |
| age | int(4) | YES | | NULL | |
| shouji | char(11) | YES | | NULL | |
| sex | char(4) | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
5、 改变字段
语法：
alter table <表名> CHANGE [COLUMN] old_col_name new_col_name column_definition
6、 修改字段类型
mysql> desc test;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id | int(4) | NO | PRI | 0 | |
| name | varchar(16) | YES | MUL | NULL | |
| age | int(4) | YES | | NULL | | 修改前
| shouji | char(11) | YES | | NULL | |
| sex | char(4) | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
mysql> alter table test modify age char(4);
Query OK, 2 rows affected (0.06 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> desc test;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id | int(4) | NO | PRI | 0 | |
| name | varchar(16) | YES | MUL | NULL | |
| age | char(4) | YES | | NULL | | 修改后
| shouji | char(11) | YES | | NULL | |
| sex | char(4) | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
