ERROR 1062 (23000): Duplicate entry '0' for key 'PRIMARY'

OS: centos 6.3
DB: 5.5.14

测试创建yoon测试表，没有主键，没有索引，基础数据内容如下：

mysql> select * from yoon;
+----+----------+------+
| id | name | user |
+----+----------+------+
| 1 | \""##!aa | NULL |
| 2 | z2 | NULL |
| 3 | z3 | NULL |
| 4 | z4 | NULL |
| 5 | z5 | NULL |
+----+----------+------+
5 rows in set (0.00 sec)

测试通过一条命令将id设为自增主键，命令alter table yoon add constraint auto_increment primary key yoon(id);创建成功，但是插入2条数据发现报错，场景如下：

mysql> desc yoon;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   |     | 0       |       |
| name  | varchar(20) | YES  |     | NULL    |       |
| user  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

mysql> select * from yoon;
+----+----------+------+
| id | name     | user |
+----+----------+------+
|  1 | \""##!aa | NULL |
|  2 | z2       | NULL |
|  3 | z3       | NULL |
|  4 | z4       | NULL |
|  5 | z5       | NULL |
+----+----------+------+
5 rows in set (0.00 sec)

mysql> show index from yoon;
Empty set (0.00 sec)

mysql> alter table yoon add constraint auto_increment primary key yoon(id);
Query OK, 0 rows affected (0.29 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from yoon;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| yoon  |          0 | PRIMARY  |            1 | id          | A         |           6 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.01 sec)

mysql> insert into yoon(name,user) values ('z','HHH'); 
Query OK, 1 row affected (0.02 sec)

mysql> select * from yoon;
+----+----------+------+
| id | name     | user |
+----+----------+------+
|  0 | z        | HHH  |
|  1 | \""##!aa | NULL |
|  2 | z2       | NULL |
|  3 | z3       | NULL |
|  4 | z4       | NULL |
|  5 | z5       | NULL |
+----+----------+------+
6 rows in set (0.01 sec)

mysql> insert into yoon(name,user) values ('z6','HHH'); 
ERROR 1062 (23000): Duplicate entry '0' for key 'PRIMARY'

mysql> delete from yoon where id=0;
Query OK, 1 row affected (0.01 sec)

mysql> show index from yoon;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| yoon  |          0 | PRIMARY  |            1 | id          | A         |           6 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

mysql> select * from yoon;
+----+----------+------+
| id | name     | user |
+----+----------+------+
|  1 | \""##!aa | NULL |
|  2 | z2       | NULL |
|  3 | z3       | NULL |
|  4 | z4       | NULL |
|  5 | z5       | NULL |
+----+----------+------+
5 rows in set (0.00 sec)

mysql> alter table yoon modify column id int auto_increment;
Query OK, 5 rows affected (0.03 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> select * from yoon;
+----+----------+------+
| id | name     | user |
+----+----------+------+
|  1 | \""##!aa | NULL |
|  2 | z2       | NULL |
|  3 | z3       | NULL |
|  4 | z4       | NULL |
|  5 | z5       | NULL |
|  6 | z6       | HHH  |
+----+----------+------+
6 rows in set (0.00 sec)

总结：主要原因alter语法使用不正确，有时候不报错，并不代表命令正确。具体语法如下：
http://dev.mysql.com/doc/refman/5.0/en/alter-table.html