mysql修改表默认字符集可能带来的隐患_MySQL
bitsCN.com
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
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

Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

AI Hentai Generator
Generate AI Hentai for free.

Hot Article

Hot Tools

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Hot Topics

Big data structure processing skills: Chunking: Break down the data set and process it in chunks to reduce memory consumption. Generator: Generate data items one by one without loading the entire data set, suitable for unlimited data sets. Streaming: Read files or query results line by line, suitable for large files or remote data. External storage: For very large data sets, store the data in a database or NoSQL.

Backing up and restoring a MySQL database in PHP can be achieved by following these steps: Back up the database: Use the mysqldump command to dump the database into a SQL file. Restore database: Use the mysql command to restore the database from SQL files.

MySQL query performance can be optimized by building indexes that reduce lookup time from linear complexity to logarithmic complexity. Use PreparedStatements to prevent SQL injection and improve query performance. Limit query results and reduce the amount of data processed by the server. Optimize join queries, including using appropriate join types, creating indexes, and considering using subqueries. Analyze queries to identify bottlenecks; use caching to reduce database load; optimize PHP code to minimize overhead.

How to insert data into MySQL table? Connect to the database: Use mysqli to establish a connection to the database. Prepare the SQL query: Write an INSERT statement to specify the columns and values to be inserted. Execute query: Use the query() method to execute the insertion query. If successful, a confirmation message will be output.

Creating a MySQL table using PHP requires the following steps: Connect to the database. Create the database if it does not exist. Select a database. Create table. Execute the query. Close the connection.

To use MySQL stored procedures in PHP: Use PDO or the MySQLi extension to connect to a MySQL database. Prepare the statement to call the stored procedure. Execute the stored procedure. Process the result set (if the stored procedure returns results). Close the database connection.

One of the major changes introduced in MySQL 8.4 (the latest LTS release as of 2024) is that the "MySQL Native Password" plugin is no longer enabled by default. Further, MySQL 9.0 removes this plugin completely. This change affects PHP and other app

Oracle database and MySQL are both databases based on the relational model, but Oracle is superior in terms of compatibility, scalability, data types and security; while MySQL focuses on speed and flexibility and is more suitable for small to medium-sized data sets. . ① Oracle provides a wide range of data types, ② provides advanced security features, ③ is suitable for enterprise-level applications; ① MySQL supports NoSQL data types, ② has fewer security measures, and ③ is suitable for small to medium-sized applications.
