就是对数据表进行列的删除和增加，约束的添加和删除
1.添加数据列
查看users1的数据表结构
show columns from users1;
mysql> show columns from users1;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(10) | NO | | NULL | |
| password | varchar(32) | NO | | NULL | |
| pid | smallint(5) unsigned | YES | MUL | NULL | |
+----------+----------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)
添加age在数据项的最后一列
alter table users1 add age tinyint unsigned not null default 10;
mysql> show columns from users1;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| username | varchar(10) | NO | | NULL | |
| password | varchar(32) | NO | | NULL | |
| pid | smallint(5) unsigned | YES | MUL | NULL | |
| age | tinyint(3) unsigned | NO | | 10 | |
+----------+----------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)
在数据项的第一项添加
alter table users1 add truename varchar(20) unsigned first;
删除单行
alter table users drop truename;
删除多行
alter table users drop passward,drop age;
同时删除和添加
alter table users1 drop truename add age tinyint unsigned not null default 10;
添加主键，作用是数据项的唯一表示符，就像id
mysql> show columns from users2;
+----------+----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| username | varchar(20) | NO | | NULL | |
| pid | smallint(5) unsigned | YES | | NULL | |
| id | smallint(5) unsigned | YES | | NULL | |
+----------+----------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
alter table users2 add constraint pk_user2_id primary key (id);
mysql> show columns from users2;
+----------+----------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| username | varchar(20) | NO | | NULL | |
| pid | smallint(5) unsigned | YES | | NULL | |
| id | smallint(5) unsigned | NO | PRI | NULL | |
+----------+----------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
2.添加字段的字符约束
alter teable users2 add id smallint unsigned;
3.添加唯一约束
alter table users2 add unique (username);
4.添加外键列
alter table users2 add foreign key (pid) references provinces (id);
5.添加默认值
alter table users2 alter age set default 15;
6.删除默认值
alter table users2 alter age drop default ;
7.删除主键约束，一个表就一个主键约束，所以不需要添加id
alter table users2 drop foreign key users2_ibfk_1;