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)
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