mysql修改表默认字符集可能带来的隐患

原文链接：http://www.javaarch.net/jiagoushi/627.htm

mysql修改表默认字符集可能带来的隐患

我们在建表后如果后续需要修改表的默认字符集，可以通过ALTER TABLE来修改表的默认编码

1. ALTER TABLE tablename DEFAULT CHARACTER SET utf8;

但是这样有个问题，只改了表定义的默认编码，对于每个列的已有字段的内容还是使用以前的编码，已有表数据不会做编码转换。

mysql> create table mybig5 (id int not null auto_increment primary key,
-> subject varchar(100) ) engine=innodb default charset big5;
Query OK, 0 rows affected (0.81 sec)

mysql> show create table mybig5;
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                     |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| mybig5 | CREATE TABLE `mybig5` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`subject` varchar(100) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=big5 |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> alter table mybig5 default charset utf8;
Query OK, 0 rows affected (0.17 sec)
Records: 0  Duplicates: 0  Warnings: 0

然后我们插入一个多字节的字符串记录到表里

mysql> INSERT INTO mybig5 VALUES (NULL, UNHEX('E7BB8FE79086'));

01:08:19  [INSERT - 0 row(s), 0.000 secs]  [Error Code: 1366, SQL State: HY000]  Incorrect string value: '\xE7\xBB\x8F\xE7\x90\x86' for column 'SUBJECT' at row 1
... 1 statement(s) executed, 0 row(s) affected, exec/fetch time: 0.000/0.000 sec  [0 successful, 0 warnings, 1 errors]

mysql> show create table mybig5;
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                                        |
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| mybig5 | CREATE TABLE `mybig5` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`subject` varchar(100) CHARACTER SET big5 DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

我们可以看到列subject还是big5编码，没有改为utf8编码。

我们可以使用2. ALTER TABLE tablename CONVERT TO CHARACTER SET utf8;来修改列的编码

mysql> show create table mybig5;
mysql> +--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-> | Table  | Create Table                                                                                                                                                     |
-> +--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-> | mybig5 | CREATE TABLE `mybig5` (
->   `id` int(11) NOT NULL AUTO_INCREMENT,
->   `subject` varchar(100) DEFAULT NULL,
->   PRIMARY KEY (`id`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
-> +--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-> 1 row in set (0.00 sec)

但是这里要注意已有内容编码转换要注意编码可转换，不会成为乱码