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MySQL数据库:SQL语句中不可省去的引号
有一次在代码中发现了一个BUG,在排查过程中,差点晕翻;作一个DBA也碰到这种低级错误;大为自嘲。其实这个问题在官方手册里就写得很明白;(可见官方文档的重要性)
mysql > create table h1 (id int , col1 char ( 1 ));Query OK, 0 rows affected ( 0.00 sec)mysql > insert into h1 select 1 ,’I ' ;Query OK, 1 row affected (0.00 sec)Records: 1 Duplicates: 0 Warnings: 0mysql> insert into h1 select 2,’F ' ;Query OK, 1 row affected ( 0.00 sec)Records: 1 Duplicates: 0 Warnings: 0 mysql > insert into h1 select 2 ,’ 0 ′;Query OK, 1 row affected ( 0.00 sec)Records: 1 Duplicates: 0 Warnings: 0mysql > select * from h1 ;+ —— + —— + | id | col1 | + —— + —— + | 1 | I | | 2 | F | | 2 | 0 | + —— + —— + 3 rows in set ( 0.00 sec)mysql > select * from h1 where col1 = 0 ;+ —— + —— + | id | col1 | + —— + —— + | 1 | I | | 2 | F | | 2 | 0 | + —— + —— + 3 rows in set , 2 warnings ( 0.00 sec)mysql > select * from h1 where col1 = ’ 0 ′;+ —— + —— + | id | col1 | + —— + —— + | 2 | 0 | + —— + —— + 1 row in set ( 0.00 sec)mysql > select * from h1 where col1 = ’ 1 ′;Empty set ( 0.00 sec)mysql > select * from h1 where col1 = 1 ;Empty set , 2 warnings ( 0.00 sec)
发现没有:
col1=0 ; 所有行被选中;
col1=’0′ ; 结果集正常
mysql > select * from h1 where col1 = ’ 1 ′;Empty set ( 0.00 sec)mysql > select * from h1 where col1 = 1 ;Empty set , 2 warnings ( 0.00 sec)
想知道为什么吗?
看这里.
mysql > show warnings;+ ——— + —— + ————————————— + | Level | Code | Message | + ——— + —— + ————————————— + | Warning | 1292 | Truncated incorrect DOUBLE value: ‘I’ | | Warning | 1292 | Truncated incorrect DOUBLE value: ‘F’ | + ——— + —— + ————————————— + 2 rows in set ( 0.00 sec)
MySQL当数据类型不匹配时,尊重用户的输入,将字段的类型做了隐式转换然后来匹配,而字符向数字转,结果都是0;
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InnoDBBufferPool reduces disk I/O by caching data and indexing pages, improving database performance. Its working principle includes: 1. Data reading: Read data from BufferPool; 2. Data writing: After modifying the data, write to BufferPool and refresh it to disk regularly; 3. Cache management: Use the LRU algorithm to manage cache pages; 4. Reading mechanism: Load adjacent data pages in advance. By sizing the BufferPool and using multiple instances, database performance can be optimized.

Compared with other programming languages, MySQL is mainly used to store and manage data, while other languages such as Python, Java, and C are used for logical processing and application development. MySQL is known for its high performance, scalability and cross-platform support, suitable for data management needs, while other languages have advantages in their respective fields such as data analytics, enterprise applications, and system programming.

MySQL is worth learning because it is a powerful open source database management system suitable for data storage, management and analysis. 1) MySQL is a relational database that uses SQL to operate data and is suitable for structured data management. 2) The SQL language is the key to interacting with MySQL and supports CRUD operations. 3) The working principle of MySQL includes client/server architecture, storage engine and query optimizer. 4) Basic usage includes creating databases and tables, and advanced usage involves joining tables using JOIN. 5) Common errors include syntax errors and permission issues, and debugging skills include checking syntax and using EXPLAIN commands. 6) Performance optimization involves the use of indexes, optimization of SQL statements and regular maintenance of databases.

MySQL is suitable for beginners to learn database skills. 1. Install MySQL server and client tools. 2. Understand basic SQL queries, such as SELECT. 3. Master data operations: create tables, insert, update, and delete data. 4. Learn advanced skills: subquery and window functions. 5. Debugging and optimization: Check syntax, use indexes, avoid SELECT*, and use LIMIT.

MySQL efficiently manages structured data through table structure and SQL query, and implements inter-table relationships through foreign keys. 1. Define the data format and type when creating a table. 2. Use foreign keys to establish relationships between tables. 3. Improve performance through indexing and query optimization. 4. Regularly backup and monitor databases to ensure data security and performance optimization.

MySQL is an open source relational database management system that is widely used in Web development. Its key features include: 1. Supports multiple storage engines, such as InnoDB and MyISAM, suitable for different scenarios; 2. Provides master-slave replication functions to facilitate load balancing and data backup; 3. Improve query efficiency through query optimization and index use.

SQL is used to interact with MySQL database to realize data addition, deletion, modification, inspection and database design. 1) SQL performs data operations through SELECT, INSERT, UPDATE, DELETE statements; 2) Use CREATE, ALTER, DROP statements for database design and management; 3) Complex queries and data analysis are implemented through SQL to improve business decision-making efficiency.

The basic operations of MySQL include creating databases, tables, and using SQL to perform CRUD operations on data. 1. Create a database: CREATEDATABASEmy_first_db; 2. Create a table: CREATETABLEbooks(idINTAUTO_INCREMENTPRIMARYKEY, titleVARCHAR(100)NOTNULL, authorVARCHAR(100)NOTNULL, published_yearINT); 3. Insert data: INSERTINTObooks(title, author, published_year)VA


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