语法：
alter table 表名 执行动作 字段名;
1. 添加到末尾
alter table 表名 add 字段名 数据类型;
例如：
mysql> desc t1;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| name | char(20) | YES | | NULL | |
+-------+----------+------+-----+---------+-------+
1 row in set (0.00 sec)
mysql> alter table t1 add age tinyint unsigned;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t1;
+-------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| name | char(20) | YES | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
2. 添加到开始
alter table 表名 add 字段名 数据类型 first;
例如：
mysql> alter table t1 add id int first;
Query OK, 0 rows affected (0.13 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t1;
+-------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | char(20) | YES | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
3. 添加到指定位置
添加到指定字段名下边
alter table 表名 add 表字段 数据类型 after 指定字段名；
例如：
mysql> alter table t1 add sex enum("M","F") after name;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t1;
+-------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | char(20) | YES | | NULL | |
| sex | enum('M','F') | YES | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
4. 删除字段
alter table 表名 drop 字段名；
例如：
mysql> alter table t1 drop sex;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t1;
+-------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | char(20) | YES | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
5. 修改数据类型
alter table 表名 modify 字段名 新的数据类型;
例如：
mysql> alter table t1 modify name varchar(10);
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t1;
+-------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(10) | YES | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+-------+
3 rows in set (0.01 sec)
6. 修改字段名
alter table 表名 change 旧名 新名 数据类型;
例如：
mysql> alter table t1 change id ID int(11);
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t1;
+-------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| ID | int(11) | YES | | NULL | |
| name | varchar(10) | YES | | NULL | |
| age | tinyint(3) unsigned | YES | | NULL | |
+-------+---------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
7. 修改表名
alter table 表名 rename 新表名;
例如：
mysql> alter table t1 rename new_t1;
Query OK, 0 rows affected (0.01 sec)