关系型数据库由哪三部分组成
 
 使用关系数据库 (Using The Relational Database)
 
 
A relational database is used for electronic data management in computer systems and is based on the relational database model. This was proposed in 1970 by Edgar F. Codd first time and is still, despite some criticism, as an established standard for databases.
 
 
 关系数据库用于计算机系统中的电子数据管理，并且基于关系数据库模型。 这是由Edgar F. Codd于1970年首次提出的，尽管有一些批评，它仍然是数据库的既定标准。 
 
 
The associated database management system is called a relational database management system, or RDBMS (Relational Database Management System). To query and manipulate the data predominantly the database language SQL (Structured Query Language) is used.
 
 
 关联的数据库管理系统称为关系数据库管理系统或RDBMS(关系数据库管理系统)。 为了主要查询和操作数据，使用了数据库语言SQL(结构化查询语言)。 
 
 
Basis of the concept of relational databases is the relation, in a sense of well-defined mathematical concept. It represents a mathematical description of a table, see database relation. Operations on these routes are determined by the relational algebra. The relational algebra is thus the theoretical basis of SQL.
 
 
 从明确定义的数学概念的意义上来说，关系数据库的概念的基础是关系。 它代表表的数学描述，请参见数据库关系。 这些路线上的运算由关系代数决定。 因此，关系代数是SQL的理论基础。 
 
 
Despite the math and abstract definition of the database model relational databases to handle relatively simple and flexible. This had great impact on the success of database technology.
 
 
 尽管对数据库模型进行了数学和抽象定义，但关系数据库仍然可以相对简单灵活地进行处理。 这对数据库技术的成功产生了重大影响。 
 
 
 关系数据库–基本概念 (Relational Database – Basic concepts)
 
 
A relational database can be thought of as a collection of tables (the relations), which are stored in records. Each row (tuple) in one table is a record (record). Each tuple is a set of attribute values (attributes = attributes), the columns of the table. The relation schema specifying, in the number and type of attributes for a relation. The picture illustrates the relation R with attributes A1 to An in the columns.
 
 
 关系数据库可以认为是表(关系)的集合，这些表存储在记录中。 一个表中的每一行(元组)都是一条记录(记录)。 每个元组是一组属性值(属性=属性)，即表的列。 关系模式，用于指定关系的属性的数量和类型。 该图在列中说明了具有属性A1到An的关系R。 
 
 
For example, one book at a library by the record (book-id, author, publisher, publishing year, title, date of recording will be described). A record must be clearly identifiable. This takes about one or more keys (English Key). In this case book contains the key ID. A key must never change. It relates to the record and not on the position in the table.
 
 
 例如，将按记录记录图书馆的一本书(书号，作者，出版者，出版年份，书名，记录日期)。 记录必须清晰可辨。 这大约需要一个或多个键(英文键)。 在这种情况下，书包含密钥ID。 钥匙永远不能改变。 它与记录有关，而不与表中的位置有关。 
 
 
 表之间的关系 (Relations between tables)
 
 
Furthermore, links are used to express the relationships between tables. A library database could therefore be implemented as follows:
 
 
 此外，链接用于表示表之间的关系。 因此，可以如下实现库数据库： 
 
 
Table books, which contains one row for each book:
 
 
 表格书籍，每本书包含一行： 
 
 
Each line consists of the columns of the table (attribute): book ID, author, publisher, publishing year, title, date of recording.
 每行包括表格的列(属性)：书籍ID，作者，出版者，出版年份，书名，记录日期。 
The key is the book ID, because it marks any book unmistakably.
 密钥是书籍ID，因为它可以清楚地标记任何书籍。 
 
The entry (10.3) would be called so that the user has with the ID 10 (“Hans bookworms’) the book with the ID3 (“My life with Asterix”). The same user also has the book “Borrowed Printing Made Easy”, which is evidenced by the table entry (10, 2). The key one here takes the attribute set (user-ID, book ID). At the same time connects the user ID each entry in the table Item out with an entry in the table users, and the book ID Item out of each entry with an entry of the table combines books. Therefore, these attributes mean in this context, foreign key (English foreign key).
 
 
 条目(10.3)将会被调用，以便用户拥有ID 10(“汉书虫”)和ID3(“ Asterix我的生活”)的书。 同一用户还拥有一本书“轻松进行借阅印刷”，这可以通过表格条目(10，2)证明。 这里的关键是采用属性集(用户ID，书ID)。 同时，将表ID中的每个条目的用户ID与表用户中的一个条目连接在一起，并将表中每个条目的用户ID条目与该表中的条目连接在一起。 因此，这些属性在此上下文中表示外键(英文外键)。 
 
 
 划界 (Demarcation)
 
 
In addition to the relational database model, there are several alternative approaches, which allow data to manage in other structures. These concepts often have only a minor importance or not yet enforced. Nevertheless, they provide an easier access for certain applications of the data to be managed.
 
 
 除了关系数据库模型外，还有几种替代方法，这些方法允许在其他结构中管理数据。 这些概念通常只具有次要的重要性，或者尚未被实施。 但是，它们为要管理的数据的某些应用程序提供了更容易的访问。 
 
 
 较旧的方法 (Older approaches)
 
 
In the 60s and 70s for operational data processing hierarchical database systems and network database systems were used. These are the data or table structure defined in the draft stage and may not vary with the query. They come in special cases, also still in use today.
 
 
 在60年代和70年代，用于操作数据处理的是分层数据库系统和网络数据库系统。 这些是在草稿阶段定义的数据或表结构，可能不会随查询而变化。 它们以特殊情况出现，今天仍在使用。 
 
 
 面向对象的数据库 (Object oriented databases)
 
 
With the advent of object-oriented programming languages object databases have been increasingly offered. This allows objects from OO languages like Java are held directly in the database – a mapping of objects to the relational table structure, the object-relational mapping is then no longer necessary. This approach has advantages over the relational design if you want to store complex data objects that can be mapped only heavy on the flat relational table structures.
 
 
 随着面向对象编程语言的出现，越来越多地提供了对象数据库。 这样就可以将诸如Java之类的OO语言的对象直接保存在数据库中-将对象映射到关系表结构，这样就不再需要对象-关系映射。 如果要存储只能在平面关系表结构上重映射的复杂数据对象，则此方法相对于关系设计具有优势。 
 
 
Object databases have, however, still disadvantages to relational databases with the processing of large amounts of data. This is caused for example by access paths to objects on multiple levels (example, inheritance and association). This results in write operations to the lock management at an exponential complexity and thus leads to poor performance. The performance issues were addressed in the object-relational databases in which only the constructs of object-oriented databases with lower complexity (eg n * log (n)) were taken.
 
 
 但是，对象数据库在处理大量数据时仍比关系数据库不利。 例如，这是由对多个级别的对象的访问路径(例如，继承和关联)引起的。 这导致对锁管理的写操作呈指数复杂性，从而导致性能下降。 在对象关系数据库中解决了性能问题，其中仅采用具有较低复杂性(例如n * log(n))的面向对象数据库的结构。 
 
 
 对象关系数据库 (Object-relational databases)
 
 
Some providers add their relational databases, object-oriented properties and then call these object-databases. However, these are not provided for the direct imaging of objects in the programming language – they are just using the concept of inheritance to simplify the definition and query of tables with similar field structures and thus their use. The SQL-99 standard was extended to object-language elements.
 
 
 一些提供程序添加他们的关系数据库，面向对象的属性，然后调用这些对象数据库。 但是，并没有为使用编程语言对对象进行直接成像提供这些功能-它们只是使用继承的概念来简化具有相似字段结构的表的定义和查询，从而简化了它们的使用。 SQL-99标准已扩展到对象语言元素。 
 
 
 半结构化数据库 (Semi-structured databases)
 
 
New concepts are the semi-structured databases. They differ from the traditional database model is that they do not have a fixed predetermined scheme. The database is hierarchically structured as a tree, and each database unit (English Entity) of the same type can have different sets of attributes.
 
 
 新概念是半结构化数据库。 它们与传统数据库模型的不同之处在于它们没有固定的预定方案。 数据库按层次结构构建为树，并且同一类型的每个数据库单元(英语实体)可以具有不同的属性集。 
 
 
Typical representative of this type are XML databases that manage the data as XML fragments. The XML data is hierarchically organized here and can contain any structure as long as they are well formed according to XML definition. The data can be queried using XQuery or XPath. Manipulation are now used proprietary language extensions. Disadvantage of current XML databases is in comparison to relational systems lower performance.
 
 
 这种类型的典型代表是将数据作为XML片段管理的XML数据库。 XML数据在这里是按层次结构组织的，并且可以包含任何结构，只要它们根据XML定义格式正确即可。 可以使用XQuery或XPath查询数据。 现在，操纵已使用专有语言扩展。 当前XML数据库的缺点是与关系系统相比性能较低。 
 
 
Semi-structured database can be implemented via extensions or server programming with relational DB where the relational model is used but no more.
 
 
 半结构化数据库可以通过关系数据库的扩展或服务器编程来实现，而关系数据库仅使用关系模型。 
 
 
Continued…
 
 
 继续… 
 
 

  
翻译自: https://www.eukhost.com/blog/webhosting/relational-database-part-1/
 
 
关系型数据库由哪三部分组成

 
关系型数据库由哪三部分组成
 
 关系数据库理论 (Theory of relational databases)
 
 
The foundations of the theory of relational database were laid by Edgar F. Codd in the 1960s and 1970s and in his work A Relational Model of Data for Large Shared Data Banks [described one]. Theoretically, all operations are based on the relational algebra.
 
 
 关系数据库理论的基础由埃德加·科德(Edgar F. Codd)在1960年代和1970年代及其著作《大型共享数据库的数据关系模型》 [已描述]中奠定。 从理论上讲，所有运算都基于关系代数。 
 
 
 了解关系代数 (Understanding the relational algebra)
 
 
The relational algebra is an algebraic model that describes how data can be stored, queried and manipulated. The main operations from which all other are derived, are the following:
 
 
 关系代数是一个代数模型，描述了如何存储，查询和操纵数据。 其他所有派生的主要操作如下： 
 
 
Projection (English projection)
 投影(英文投影) 
Selection (English selection)
 选择(英文选择) 
Cross product or Cartesian Product (Data Sheet cross product, cross join or cartesian product)
 叉积或笛卡尔积(数据表叉积，叉联接或笛卡尔积) 
Changes (English rename)
 更改(英文重命名) 
Association (English union)
 协会(英国工会) 
Difference (English set difference or minus)
 差异(英文设置的差异或减号) 
 
All requests that are made using SQL on a relational database, the database management system shown on these operators, that is translated. In practice, there are other operators, such as the join operator, but is also only a combination of cross product, selection and projection.
 
 
 使用关系数据库上SQL进行的所有请求(这些运算符上显示的数据库管理系统)均已翻译。 实际上，还有其他运算符，例如join运算符，但也只是叉积，选择和投影的组合。 
 
 
 关系代数的限制 (Restrictions on the relational algebra)
 
 
The relational algebra does not provide support for calculation of recursive queries (Transitive Closure). This means, for instance, that it is not possible in a query all ancestors of a person to calculate where they are stored in a relation person and connected by a relationship. The ancestors can only be determined through a series of questions.
 
 
 关系代数不提供对递归查询(传递闭包)的计算的支持。 例如，这意味着不可能在查询中计算一个人的所有祖先在哪里存储他们在某个关系人中并通过关系连接的位置。 只能通过一系列问题来确定祖先。 
 
 
With the introduction of SQL-99 but also an extended Relational Algebra was introduced, which allows an operation to calculate the transitive closure.
 
 
 随着SQL-99的引入，还引入了扩展的关系代数，该关系代数允许操作来计算传递闭包。 
 
 
 数据库架构和建模 (Database schema and Modeling)
 
 
An important part of relational database is its schema. The schema defines what data is stored in the database and how these data are related to each other. The process for creating a schema is called data modeling.
 
 
 关系数据库的重要部分是其架构。 该模式定义了哪些数据存储在数据库中以及这些数据如何相互关联。 创建模式的过程称为数据建模。 
 
 
For modeling of relational databases and the Entity-Relationship model is used. It is used to design a conceptual schema, which is using a database management system (DBMS) can be implemented. This step is called the logical design or data model mapping and, as a result of a database schema in the implementation of the DBMS data model.
 
 
 对于关系数据库的建模，使用了实体关系模型。 它用于设计概念性架构，该架构可使用数据库管理系统(DBMS)来实现。 此步骤被称为逻辑设计或数据模型映射，并且由于执行DBMS数据模型中的数据库模式而被称为。 
 
 
An important step in the modeling process is normalization. This will reduce redundancies and prevent anomalies in order to simplify the maintenance of a database, and ensure the consistency of the data. , Edgar F. Codd has proposed along with four normal forms, thereafter in the relational database design used and were supplemented by another.
 
 
 建模过程中的一个重要步骤是规范化。 这将减少冗余并防止异常，以简化数据库的维护并确保数据的一致性。 ，埃德加·科德(Edgar F. Codd)提出了四种范式，此后在所使用的关系数据库设计中加以补充。 
 
 
 对关系数据库模型的批评 ： (Criticism of the relational database model:)
 
 
Segmentation
 
 
 分割 
 
 
In the relational representation is the storage of an object is segmented in many different relationships. The application objects are usually complex, there are so themselves back from objects, or lists of objects. Since the relational model only consisting of values, must use complex objects in a query by the DBMS are restored by means of numerous joins from the individual relations.
 
 
 在关系表示中，对象的存储被划分为许多不同的关系。 应用程序对象通常很复杂，因此它们本身是从对象或对象列表中返回的。 由于关系模型仅由值组成，因此必须在DBMS的查询中使用复杂的对象，这是通过从各个关系中进行大量联接来恢复的。 
 
 
This can lead to confusing queries that need to be reviewed each structural modification of the application object needs to adjust down. The use of joins, which are supported by each well-matching database indexes must, makes the object access expensive than, for example in an object database, both the resource requirements as well as the development effort.
 
 
 这可能导致混淆的查询，需要对应用程序对象的每个结构修改进行下调。 每个完全匹配的数据库索引支持的联接必须使对象访问比例如对象数据库中的资源需求和开发工作都昂贵。 
 
 
Artificial key attributes
 
 
 人工关键属性 
 
 
To uniquely identify tuples in some cases, artificial keys are used. This, for example, helps to reduce the size of the key, if it is to be used as foreign keys, or to one-to-implement relations. There are therefore included in the relation attributes that have nothing to do with the abstract description of an application object nothing but “just” management information.
 
 
 为了在某​​些情况下唯一地标识元组，使用了人工密钥。 例如，这将有助于减小密钥的大小(如果将其用作外键)或实现一对一的关系。 因此，在关系属性中包含了与应用程序对象的抽象描述无关的东西，只不过是“公正”的管理信息。 
 
 
External API
 
 
 外部API 
 
 
Since in many relational databases only data manipulation languages are available with reduced thickness, is usually necessary to powerful programming interfaces. This link leads to an unfavorable handling if necessary, for example, if the quantity-SQL to process in the satzorientierten C + +, refer to impedance mismatch.
 
 
 由于在许多关系数据库中，只有可用的数据操作语言才能减小厚度，因此对于强大的编程接口通常是必需的。 该链接在必要时导致不利的处理，例如，如果要在饱和C ++中处理的数量SQL表示阻抗不匹配。 
 
 
However, there are also relational database with powerful languages such PL / SQL on Oracle or PL / pgSQL PostgreSQL, the latter turn enables the server programming with other languages such as PHP, Tcl or Python.
 
 
 但是，也有关系数据库具有强大的语言，例如Oracle上的PL / SQL或PL / pgSQL PostgreSQL，后者使服务器能够使用其他语言(例如PHP，Tcl或Python)进行编程。 
 
 
Object properties and behaviors are often not mapped
 
 
 对象属性和行为通常不映射 
 
 
In the relational database application type and the behavior of an object are not described. This description may therefore only take place outside the database in a software application. When multiple applications use the same data can lead to a redundant implementation.
 
 
 在关系数据库应用程序中，未描述对象的类型和行为。 因此，此描述只能在软件应用程序中的数据库外部进行。 当多个应用程序使用相同的数据时，可能导致冗余的实现。 
 
 
Relational Design Theory
 
 
 关系设计理论 
 
 
The relational design theory focuses on the basis of formal methods with the conceptual design of the schemas of relational databases. The relational design theory thus provides a theoretical basis for the design of a “good” relation “schemas”.
 
 
 关系设计理论侧重于形式方法的基础，以及关系数据库模式的概念设计。 因此，关系设计理论为“良好”关系“方案”的设计提供了理论基础。 
 
 
Basis for the design of a schema make it functional dependencies – more informal – are a generalization of the key concept, which aims to help make certain stored data clearly identified. Furthermore, the canonical cover of functional dependencies and multi-valued dependencies under investigation, the latter in turn are a generalization of functional dependencies.
 
 
 模式设计的基础使功能依赖关系(更非正式)是关键概念的概括，其目的是帮助明确识别某些存储的数据。 此外，正在研究的功能依赖关系和多值依赖关系的规范覆盖，后者又是功能依赖关系的概括。 
 
 
Using the functional dependencies are defined normal forms for relational schemas and are, in a “quality criterion is” is assessed on the basis of which the quality of relation schemes, quality is a collective term for properties such as manageability, comprehensibility, clarity, etc. try the relational design theory, this helps to formalize using the normal forms.
 
 
 使用功能依赖关系为关系模式定义了标准形式，并在“质量标准被评估”的基础上评估了关系方案的质量，质量是诸如可管理性，可理解性，清晰度等属性的总称。尝试关系设计理论，这有助于规范形式的形式化。 
 
 
A “good” relational schema is characterized for example by the fact that information is not stored implicitly, that information is not redundant and that there are no inconsistencies, induced by modification, deletion and addition. These undesirable properties of relation schemas are often caused by mixing, cutting, or repeated storing the identified information needs analysis in the entities.
 
 
 “良好”的关系模式的特点是，信息不是隐式存储的，信息不是冗余的，并且不存在由于修改，删除和添加而引起的不一致。 关系模式的这些不希望有的属性通常是由于在实体中混合，剪切或重复存储已标识的信息需求分析而引起的。 
 
 
A distinction in the relational design theory, two approaches:
 
 
 关系设计理论有两种区别： 
 
 
The normalization of relations will help to improve a given design: are not satisfied for a certain normal form relational schema, we can break this by using the appropriate normalization algorithms to several schemes that meet the desired normal form.
 关系的规范化将有助于改善给定的设计：对某种规范形式的关系方案不满意，我们可以通过对满足期望规范形式的几种方案使用适当的规范化算法来打破这种局面。 
The synthesis of relations aims to design an optimal “Relationalschemas”.
 关系的综合旨在设计最佳的“关系方案”。 
 
To be continued…
 
 
 未完待续… 
 
 
Study: From Wikipedia, the free encyclopedia. The text is available under the Creative Commons.
 
 
 研究：来自维基百科，免费的百科全书。 该文本可在“ 知识共享”下找到 。 
 
 

  
翻译自: https://www.eukhost.com/blog/webhosting/relational-database-part-2/
 
 
关系型数据库由哪三部分组成

关系模型的三个组成部分，是指关系数据模型的数据结构、关系数据模型的操作集合和关系数据模型的完整性约束。
 
关系数据模型的数据结构
 主要描述数据的类型、内容、性质以及数据间的联系等，是目标类型的集合。
 目标类型是数据库的祖成成分，一般可分为两类：数据类型、数据类型之间的联系。
关系数据模型的操作集合
 数据模型中数据操作主要描述在相应的数据结构上的操作类型和操作方式。它是操作算符的集合，包括若干操作和推理准则，用以对目标类型的有效实例所组成的数据库进行操作。
关系数据模型的完整性约束
 数据模型中的数据约束主要描述数据结构内数据间的语法、词义联系、他们之间的制约和依存关系，以及数据动态变化的规则，以保证数据的正确、有效和相容。它是完整性规则的集合，用以限定符合数据模型的数据库状态，以及状态的变化。
 约束条件可以按不同的原则划分为数据值的约束和数据间联系的约束；静态约束和动态约束；实体约束和实体间的参照约束等
 
数据模型按不同的应用层次分成三种类型：分别是概念数据模型、逻辑数据模型、物理数据模型。
 1、概念模型
 概念模型是一种面向用户、面向客观世界的模型，主要用来描述世界的概念化结构，它是数据库的设计人员在设计的初始阶段，摆脱计算机系统及DBMS的具体技术问题。
 概念模型用于信息世界的建模，一方面应该具有较强的语义表达能力，能够方便直接表达应用中的各种语义知识，另一方面它还应该简单、清晰、易于用户理解。
 2、逻辑模型
 逻辑模型是一种面向数据库系统的模型，是具体的DBMS所支持的数据模型，如网状数据模型(Network Data Model)、层次数据模型(Hierarchical Data Model)等等。此模型既要面向用户，又要面向系统，主要用于数据库管理系统（DBMS）的实现。
 3、物理模型
 物理模型是一种面向计算机物理表示的模型，描述了数据在储存介质上的组织结构，它不但与具体的DBMS有关，而且还与操作系统和硬件有关。
 每一种逻辑数据模型在实现时都有其对应的物理数据模型。DBMS为了保证其独立性与可移植性，大部分物理数据模型的实现工作由系统自动完成，而设计者只设计索引、聚集等特殊结构。

 
 
(1)关系数据结构 
 (2)关系操作集合 
 (3)关系完整性约束