选择合适的数据类型
CHAR与VARCHAR
- 存储字符串,保存和检索方式不同,CHAR固定长度,VARCHAR可变长度。
- 对比图:
- 严格模式下,若实际值超出字段定义长度,将会抛出错误。
- 范例:
-- 建表mysql> CREATE TABLE vc (v VARCHAR(4), c CHAR(4));Query OK, 0 rows affected (0.29 sec)-- 插入数据mysql> INSERT INTO vc VALUES('ab ', 'ab ');Query OK, 1 row affected (0.09 sec)-- 查询mysql> SELECT CONCAT(v, '+'), CONCAT(c, '+') FROM vc;+----------------+----------------+| CONCAT(v, '+') | CONCAT(c, '+') |+----------------+----------------+| ab + | ab+ |+----------------+----------------+1 row in set (0.06 sec)
- CHAR比VARCHAR速度快得多,但浪费存储空间。随着MySQL对VARCHAR的性能提升,很多应用也经常用VARCHAR。
- 不同存储引擎对CHAR,VARCHAR的使用原则有所不同:
TEXT与BLOB
- BLOB可保存二进制数据,TEXT只能保存字符串数据。
- BLOB和TEXT会带来一些性能问题,特别是删除操作。删除会留下一些空洞,对以后插入数据到该空洞会有影响,可定期使用OPTIMIZE TABLE进行优化。如,
mysql> CREATE TABLE blob_test( -> id INT NOT NULL AUTO_INCREMENT, -> content TEXT, -> PRIMARY KEY (id))ENGINE=MyISAM;Query OK, 0 rows affected (0.34 sec)
插入一些数据后,数据文件大小为:
mysql> SELECT count(1) FROM blob_test;+----------+| count(1) |+----------+| 12288 |+----------+存储文件大小此时大于7M:
我们删除一些数据后,文件大小并没有变:
mysql> DELETE FROM blob_test WHERE id > 5000;Query OK, 7288 rows affected (0.16 sec)

当我们执行优化OPTIMIZE TABLE blob_test后,文件大小才缩小:
mysql> OPTIMIZE TABLE blob_test;+---------------------+----------+----------+----------+| Table | Op | Msg_type | Msg_text |+---------------------+----------+----------+----------+| mysqltest.blob_test | optimize | status | OK |+---------------------+----------+----------+----------+1 row in set (0.08 sec)
- 可以使用合成的(Synthetic)索引来提高大文本字段(BLOB或TEXT)的查询性能。
- 在不必要的时候避免检索大型的BLOG或TEXT值。
- 可以把BLOG或TEXT列分离到单独的表中。这会加少主表的碎片。
浮点数与定点数
- 对于浮点数,如果插入的值的精度超过定义的精度,则会四舍五入;如果SQLMode为Tranditional,若插入值的精度大于定义的精度,则会报错。
- 定点数与浮点数不一样。定点数以字符串存储,精度更高。
- 使用浮点数和定点数的几个原则:
1. 浮点数存在误差问题;
2. 对货币等对精度敏感的数据,应采用定点数来表示或存储;
3. 若程序中用到浮点数,要注意其误差问题,尽量避免浮点数比较;
4. 注意浮点数中一些特殊值的处理。
日期类型选择
- 对于日期类型的选择在前面的文章也详述过,这里作个简单的总结:
不吝指正。

MySQLoffersvariousstorageengines,eachsuitedfordifferentusecases:1)InnoDBisidealforapplicationsneedingACIDcomplianceandhighconcurrency,supportingtransactionsandforeignkeys.2)MyISAMisbestforread-heavyworkloads,lackingtransactionsupport.3)Memoryengineis

Common security vulnerabilities in MySQL include SQL injection, weak passwords, improper permission configuration, and unupdated software. 1. SQL injection can be prevented by using preprocessing statements. 2. Weak passwords can be avoided by forcibly using strong password strategies. 3. Improper permission configuration can be resolved through regular review and adjustment of user permissions. 4. Unupdated software can be patched by regularly checking and updating the MySQL version.

Identifying slow queries in MySQL can be achieved by enabling slow query logs and setting thresholds. 1. Enable slow query logs and set thresholds. 2. View and analyze slow query log files, and use tools such as mysqldumpslow or pt-query-digest for in-depth analysis. 3. Optimizing slow queries can be achieved through index optimization, query rewriting and avoiding the use of SELECT*.

To monitor the health and performance of MySQL servers, you should pay attention to system health, performance metrics and query execution. 1) Monitor system health: Use top, htop or SHOWGLOBALSTATUS commands to view CPU, memory, disk I/O and network activities. 2) Track performance indicators: monitor key indicators such as query number per second, average query time and cache hit rate. 3) Ensure query execution optimization: Enable slow query logs, record and optimize queries whose execution time exceeds the set threshold.

The main difference between MySQL and MariaDB is performance, functionality and license: 1. MySQL is developed by Oracle, and MariaDB is its fork. 2. MariaDB may perform better in high load environments. 3.MariaDB provides more storage engines and functions. 4.MySQL adopts a dual license, and MariaDB is completely open source. The existing infrastructure, performance requirements, functional requirements and license costs should be taken into account when choosing.

MySQL uses a GPL license. 1) The GPL license allows the free use, modification and distribution of MySQL, but the modified distribution must comply with GPL. 2) Commercial licenses can avoid public modifications and are suitable for commercial applications that require confidentiality.

The situations when choosing InnoDB instead of MyISAM include: 1) transaction support, 2) high concurrency environment, 3) high data consistency; conversely, the situation when choosing MyISAM includes: 1) mainly read operations, 2) no transaction support is required. InnoDB is suitable for applications that require high data consistency and transaction processing, such as e-commerce platforms, while MyISAM is suitable for read-intensive and transaction-free applications such as blog systems.

In MySQL, the function of foreign keys is to establish the relationship between tables and ensure the consistency and integrity of the data. Foreign keys maintain the effectiveness of data through reference integrity checks and cascading operations. Pay attention to performance optimization and avoid common errors when using them.


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