MYSQL表的添加与删除字段
+---------+-----------------------+------+-----+---------+-------+
| Field   | Type                  | Null | Key | Default | Extra |
+---------+-----------------------+------+-----+---------+-------+
| tagname | char(20)              | NO   | PRI | NULL    |       |
| closed  | tinyint(1)            | NO   | MUL | 0       |       |
| total   | mediumint(8) unsigned | NO   | MUL | NULL    |       |
+---------+-----------------------+------+-----+---------+-------+
添加一列自增的ID，如何添加？

alter table cdb_tags add id int(6) default 0 not null auto_increment,add primary key(id);
ERROR 1067 (42000): Invalid default value for 'id'

alter table cdb_tags add id int(6) auto_increment,add primary key(id);
ERROR 1068 (42000): Multiple primary key defined
我们可以看出tagname是主键，所以不能再添加主键了。

-------定义为unique key就可以了----
alter table cdb_tags add id int(6) auto_increment,add unique key(id);
Query OK, 8 rows affected (0.05 sec)
Records: 8  Duplicates: 0  Warnings: 0
-----------------------------------
mysql> describe cdb_tags;
+---------+-----------------------+------+-----+---------+----------------+
| Field   | Type                  | Null | Key | Default | Extra          |
+---------+-----------------------+------+-----+---------+----------------+
| tagname | char(20)              | NO   | PRI | NULL    |                |
| closed  | tinyint(1)            | NO   | MUL | 0       |                |
| total   | mediumint(8) unsigned | NO   | MUL | NULL    |                |
| id      | int(6)                | NO   | UNI | NULL    | auto_increment |
+---------+-----------------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)

------删除id字段OK----------------
alter table cdb_tags drop id;
Query OK, 8 rows affected (0.08 sec)
Records: 8  Duplicates: 0  Warnings: 0
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