数据库三大范式最简单的解释
关系数据库中的关系必须满足一定的要求，即满足不同的范式。目前关系数据库有六种范式：第一范式（1NF）、第二范式（2NF）、第三范式（3NF）、第四范式（4NF）、第五范式（5NF）和第六范式（6NF）。满足最低要求的范式是第一范式（1NF）。在第一范式的基础上
关系数据库中的关系必须满足一定的要求，即满足不同的范式。目前关系数据库有六种范式：第一范式（1NF）、第二范式（2NF）、第三范式（3NF）、第四范式（4NF）、第五范式（5NF）和第六范式（6NF）。满足最低要求的范式是第一范式（1NF）。在第一范式的基础上进一步满足更多要求的称为第二范式（2NF），其余范式以次类推。一般说来，数据库只需满足第三范式（3NF）就行了。
很多资料上的范式都讲的很难理解，这里总结一下三大范式，便于读者简易的理解。
1NF：字段是原子性的，不可分;
2NF：有主键，非主键字段依赖主键。确保一个表只说明一个事物
3NF：非主键字段不能相互依赖。 每列都与主键有直接关系，不存在传递的依赖
第一范式：不符合第一范式的例子：
表：字段1、 字段2(字段2.1、字段2.2)、字段3 ......
第二范式：不符合第二范式的例子:
表：学号、课程号、姓名、学分;
这个表明显说明了两个事务:学生信息, 课程信息;由于非主键字段必须依赖主键，这里学分依赖课程号，姓名依赖与学号，所以不符合二范式。
第三范式：不符合第三范式的例子:
表：学号、姓名、 年龄、 所在学院、学院联系电话、学院联系电话
存在依赖传递: (学号) → (所在学院) → (学院地点, 学院电话)
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
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
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
How does Go language implement the addition, deletion, modification and query operations of the database?
Mar 27, 2024 pm 09:39 PM
Go language is an efficient, concise and easy-to-learn programming language. It is favored by developers because of its advantages in concurrent programming and network programming. In actual development, database operations are an indispensable part. This article will introduce how to use Go language to implement database addition, deletion, modification and query operations. In Go language, we usually use third-party libraries to operate databases, such as commonly used sql packages, gorm, etc. Here we take the sql package as an example to introduce how to implement the addition, deletion, modification and query operations of the database. Assume we are using a MySQL database.
iOS 18 adds a new 'Recovered' album function to retrieve lost or damaged photos
Jul 18, 2024 am 05:48 AM
Apple's latest releases of iOS18, iPadOS18 and macOS Sequoia systems have added an important feature to the Photos application, designed to help users easily recover photos and videos lost or damaged due to various reasons. The new feature introduces an album called "Recovered" in the Tools section of the Photos app that will automatically appear when a user has pictures or videos on their device that are not part of their photo library. The emergence of the "Recovered" album provides a solution for photos and videos lost due to database corruption, the camera application not saving to the photo library correctly, or a third-party application managing the photo library. Users only need a few simple steps
How does Hibernate implement polymorphic mapping?
Apr 17, 2024 pm 12:09 PM
Hibernate polymorphic mapping can map inherited classes to the database and provides the following mapping types: joined-subclass: Create a separate table for the subclass, including all columns of the parent class. table-per-class: Create a separate table for subclasses, containing only subclass-specific columns. union-subclass: similar to joined-subclass, but the parent class table unions all subclass columns.
Detailed tutorial on establishing a database connection using MySQLi in PHP
Jun 04, 2024 pm 01:42 PM
How to use MySQLi to establish a database connection in PHP: Include MySQLi extension (require_once) Create connection function (functionconnect_to_db) Call connection function ($conn=connect_to_db()) Execute query ($result=$conn->query()) Close connection ( $conn->close())
How to handle database connection errors in PHP
Jun 05, 2024 pm 02:16 PM
To handle database connection errors in PHP, you can use the following steps: Use mysqli_connect_errno() to obtain the error code. Use mysqli_connect_error() to get the error message. By capturing and logging these error messages, database connection issues can be easily identified and resolved, ensuring the smooth running of your application.
An in-depth analysis of how HTML reads the database
Apr 09, 2024 pm 12:36 PM
HTML cannot read the database directly, but it can be achieved through JavaScript and AJAX. The steps include establishing a database connection, sending a query, processing the response, and updating the page. This article provides a practical example of using JavaScript, AJAX and PHP to read data from a MySQL database, showing how to dynamically display query results in an HTML page. This example uses XMLHttpRequest to establish a database connection, send a query and process the response, thereby filling data into page elements and realizing the function of HTML reading the database.
How to connect to remote database using Golang?
Jun 01, 2024 pm 08:31 PM
Through the Go standard library database/sql package, you can connect to remote databases such as MySQL, PostgreSQL or SQLite: create a connection string containing database connection information. Use the sql.Open() function to open a database connection. Perform database operations such as SQL queries and insert operations. Use defer to close the database connection to release resources.
How to use database callback functions in Golang?
Jun 03, 2024 pm 02:20 PM
Using the database callback function in Golang can achieve: executing custom code after the specified database operation is completed. Add custom behavior through separate functions without writing additional code. Callback functions are available for insert, update, delete, and query operations. You must use the sql.Exec, sql.QueryRow, or sql.Query function to use the callback function.
