Home Database Mysql Tutorial 数据库中的第1、2、3范式 (昨天没睡好,因为那个蚊子~~)

数据库中的第1、2、3范式 (昨天没睡好,因为那个蚊子~~)

Jun 07, 2016 pm 03:36 PM
database yesterday paradigm mosquito

半夜醒来,有个蚊子在我旁边嗡嗡,打N下没弄死,自己身上倒是不少巴掌印,抑郁到天亮。。。 1 第一范式(1NF) 在任何一个关系数据库中,第一范式(1NF)是对关系模式的基本要求,不满足第一范式(1NF)的数据库就不是关系数据库。 所谓第一范式(1NF)是指

半夜醒来,有个蚊子在我旁边嗡嗡,打N下没弄死,自己身上倒是不少巴掌印,抑郁到天亮。。。


1 第一范式(1NF)
  在任何一个关系数据库中,第一范式(1NF)是对关系模式的基本要求,不满足第一范式(1NF)的数据库就不是关系数据库。
  所谓第一范式(1NF)是指数据库表的每一列都是不可分割的基本数据项,同一列中不能有多个值,即实体中的某个属性不能有多个值或者不能有重复的属 性。如果出现重复的属性,就可能需要定义一个新的实体,新的实体由重复的属性构成,新实体与原实体之间为一对多关系。在第一范式(1NF)中表的每一行只 包含一个实例的信息。简而言之,第一范式就是无重复的列。


2 第二范式(2NF)
  第二范式(2NF)是在第一范式(1NF)的基础上建立起来的,即满足第二范式(2NF)必须先满足第一范式(1NF)。第二范式(2NF)要求数据 库表中的每个实例或行必须可以被唯一地区分。为实现区分通常需要为表加上一个列,以存储各个实例的唯一标识。这个唯一属性列被称为主关键字或主键、主码。
  第二范式(2NF)要求实体的属性完全依赖于主关键字。所谓完全依赖是指不能存在仅依赖主关键字一部分的属性,如果存在,那么这个属性和主关键字的这 一部分应该分离出来形成一个新的实体,新实体与原实体之间是一对多的关系。为实现区分通常需要为表加上一个列,以存储各个实例的唯一标识。简而言之,第二 范式就是非主属性非部分依赖于主关键字。

所有单关键字的数据库表都符合第二范式,因为不可能存在组合关键字


3 第三范式(3NF)
  满足第三范式(3NF)必须先满足第二范式(2NF)。简而言之,第三范式(3NF)要求一个数据库表中不包含已在其它表中已包含的非主关键字信息。简而言之,第三范式就是属性不依赖于其它非主属性。


Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

AI Hentai Generator

AI Hentai Generator

Generate AI Hentai for free.

Hot Article

R.E.P.O. Energy Crystals Explained and What They Do (Yellow Crystal)
2 weeks ago By 尊渡假赌尊渡假赌尊渡假赌
Repo: How To Revive Teammates
1 months ago By 尊渡假赌尊渡假赌尊渡假赌
Hello Kitty Island Adventure: How To Get Giant Seeds
4 weeks ago By 尊渡假赌尊渡假赌尊渡假赌

Hot Tools

Notepad++7.3.1

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Zend Studio 13.0.1

Powerful PHP integrated development environment

Dreamweaver CS6

Dreamweaver CS6

Visual web development tools

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)

How does Go language implement the addition, deletion, modification and query operations of the database? 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.

Detailed tutorial on establishing a database connection using MySQLi in PHP 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 does Hibernate implement polymorphic mapping? 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.

iOS 18 adds a new 'Recovered' album function to retrieve lost or damaged photos 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

An in-depth analysis of how HTML reads the database 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.

Analysis of the basic principles of MySQL database management system Analysis of the basic principles of MySQL database management system Mar 25, 2024 pm 12:42 PM

Analysis of the basic principles of the MySQL database management system MySQL is a commonly used relational database management system that uses structured query language (SQL) for data storage and management. This article will introduce the basic principles of the MySQL database management system, including database creation, data table design, data addition, deletion, modification, and other operations, and provide specific code examples. 1. Database Creation In MySQL, you first need to create a database instance to store data. The following code can create a file named "my

How to determine whether a date is yesterday in Go language? How to determine whether a date is yesterday in Go language? Mar 25, 2024 am 09:45 AM

How to determine whether a date is yesterday in Go language? In development, date processing and comparison are often involved. Sometimes it is necessary to determine whether a date is yesterday. In Go language, we can implement this function through some methods. The following will introduce how to determine whether the date is yesterday in Go language and provide specific code examples. First, we need to import the time package to manipulate dates and times. In the Go language, time is represented by the time.Time type. Next, we need to get the current time and the time to be compared

Tips and practices for handling Chinese garbled characters in databases with PHP Tips and practices for handling Chinese garbled characters in databases with PHP Mar 27, 2024 pm 05:21 PM

PHP is a back-end programming language widely used in website development. It has powerful database operation functions and is often used to interact with databases such as MySQL. However, due to the complexity of Chinese character encoding, problems often arise when dealing with Chinese garbled characters in the database. This article will introduce the skills and practices of PHP in handling Chinese garbled characters in databases, including common causes of garbled characters, solutions and specific code examples. Common reasons for garbled characters are incorrect database character set settings: the correct character set needs to be selected when creating the database, such as utf8 or u

See all articles