1.属性和域 在现实世界中,一个事物常常取若干特性来描述,这些特性成为属性(Attribute)。每个属性的取值范围对应一个值的集合,成为该属性的域(domain)。一般在关系数据模型中,限制所有的域都是原子数据(atomic data)。例如,整数、字符串是原子数据
1.属性和域
在现实世界中,一个事物常常取若干特性来描述,这些特性成为属性(Attribute)。每个属性的取值范围对应一个值的集合,成为该属性的域(domain)。一般在关系数据模型中,限制所有的域都是原子数据(atomic data)。例如,整数、字符串是原子数据,而集合、记录、数组是非原子数据。关系数据模型的这种限制称为第一范式(First Normal Form,1NF)条件。
2.主要术语
- 目或度(Degree):属性个数n是关系的目或度。
- 候选码(Candidate Key):若关系中某一属性(或属性组)的值能唯一地标识一个元组,则称该属性(属性组)为候选码。
- 主码(Primary Key):若一个关系能多个候选码,则选定其中一个为主码。
- 主属性(Key attribute):包含在任何候选码中的属性称为主属性。
- 非码属性(Non-Key attribute):不包含在任何候选码中的属性称为非码属性。
- 外码(Foreign Key):如果关系模式R中的属性(属性组)不是该关系的码,但它是其他关系的码,那么该属性(属性组)对关系模式R而言是外码。例如,客户与货款之间的借贷联系c-l(c-id, loan-no),属性c-id是客户关系的码,所以c-id是外码;属性loan-no是贷款关系中的码,所以loan-no也是外码。
- 全码(All-Key):关系模型的所有属性组是这个关系模型的候选码,称为全码。
例如,关系模式R(T,C,S),属性T表示教师,属性C表示课程,属性S表示学生。假设一个教师可以讲授多门课程,某门课程可以由多个教师讲授,学生可以听不同教师讲授的不同的课程,那么,要想区分关系中的每一个元组,这个关系模式R的码应为全属性T,C和S,即All-Key。
3.关系的性质
一个基本关系具有以下5条性质。
- 分量必须取原子值,每个分量必须是不可再分的数据项。
- 列是同质的,每列中的分量必须是同一类型的数据,来自同一个域。
- 属性不能重名。
- 行列的顺序无关。
- 任何两个元组不能完全相同,这是由主码约束来保证的。但是有些数据库若用户没有定义完整性约束条件,允许有两行以上的相同的元组。
4.关系的三种类型
- 基本关系(通常又称为基本表或基表)。是实际存在的表,它是实际存储数据的逻辑表示。
- 查询表。查询结果对应的表。
- 视图表。是由基本表或其他视图表导出的表。由于它本身不独立存储在数据库中,数据库中只存放它的定义,所以常称为虚表。

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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