alter table grade add age4 int(10); 发出警告

Question

alter table grade add age4 int(10);
发出警告
添加int    忽略了（10）

我看书上都是直接添加成功的

mysql> desc grade;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | double      | YES  |     | NULL    |       |
| username | varchar(20) | YES  |     | NULL    |       |
| grade    | float       | YES  |     | NULL    |       |
| age      | int         | YES  |     | NULL    |       |
| age1     | int         | YES  |     | NULL    |       |
| age2     | int         | YES  |     | NULL    |       |
| age3     | int         | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
7 rows in set (0.00 sec)

mysql> alter table grade add age4 int(10);
Query OK, 0 rows affected, 1 warning (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 1

mysql> alter table grade add age4 int;
ERROR 1060 (42S21): Duplicate column name 'age4'
mysql> desc grade;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | double      | YES  |     | NULL    |       |
| username | varchar(20) | YES  |     | NULL    |       |
| grade    | float       | YES  |     | NULL    |       |
| age      | int         | YES  |     | NULL    |       |
| age1     | int         | YES  |     | NULL    |       |
| age2     | int         | YES  |     | NULL    |       |
| age3     | int         | YES  |     | NULL    |       |
| age4     | int         | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
8 rows in set (0.00 sec)

Answer 1

警告应该是因为你没加默认值信息和描述信息，第一个命令已经成功添加了age4列，第二个命令再添加因为已经存在age4列就报错了