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