【免费】数据库相关基础介绍（经典）资源-CSDN文库

需积分: 0 109 浏览量 2009-12-17 01:52:45 上传评论收藏 1.64MB PDF 举报

资源详情

资源评论

资源推荐

The Entity-Relationship Model-Toward a

Unified View of Data

PETER PIN-SHAN CHEN

Massachusetts Institute of Technology

A data model, called the entity-relationship model, is proposed. This model incorporates some of

the important semantic information about the real world. A special diagrammatic technique is

introduced as a tool for database design. An example of database design and description using

the model and the diagrammatic technique is given. Some implications for data integrity, infor-

mation retrieval, and data manipulation are discussed.

The entity-relationship model can be used as a basis for unification of different views of data:

t,he network model, the relational model, and the entity set model. Semantic ambiguities in these

models are analyzed. Possible ways to derive their views of data from the entity-relationship

model are presented.

Key Words and Phrases: database design, logical view of data, semantics of data, data models,

entity-relationship model, relational model, Data Base Task Group, network model, entity set

model, data definition and manipulation, data integrity and consistency

CR Categories: 3.50, 3.70, 4.33, 4.34

1. INTRODUCTION

The logical view of data has been an important issue in recent years. Three major

data models have been proposed: the network model [2, 3, 71, the relational model

[S), and the entity set model [25]. These models have their own strengths and

weaknesses. The network model provides a more natural view of data by separating

entities and relationships (to a certain extent), but its capability to achieve data

independence has been challenged [S]. The relational model is based on relational

theory and can achieve a high degree of data independence, but it may lose some

important semantic information about the real world [12, 15, 231. The entity set

model, which is based on set theory, also achieves a high degree of data inde-

pendence, but its viewing of values such as “3” or “red” may not be natural to

some people [25].

This paper presents the entity-relationship model, which has most of the ad-

vantages of the above three models. The entity-relationship model adopts the

more natural view that the real world consists of entities and relationships. It

but not for profit; all or part of this material is granted provided that ACM’s copyright notice is

given and that reference is made to the publication, to its date of issue, and to the fact that

reprinting privileges were granted

permission of the Association for Computing Machinery.

A version of this paper was presented at the International Conference on Very Large Data Bases,

Framingham, Mass., Sept. 22-24, 1975.

Author’s address: Center for Information System Research, Alfred P. Sloan School of Manage-

ment, Massachusetts Institute of Technology, Cambridge, MA 02139.

ACM Transactions on Database Systems, Vol. 1, No. 1. March 1976, Pages 9-36.

10 - P. P.-S. Chen

incorporates some of the important semantic information about the real world

(other work in database semantics can be found in [l, 12, 15, 21, 23, and 29)).

The model can achieve a high degree of data independence and is based on set

theory and relation theory,

The entity-relationship model can be used as a basis for a unified view of data.

Most Ivork in the past has emphasized the difference between the network model

and the relational model [22]. Recently, several attempts have been made to

reduce the differences of the three data models [4, 19, 26, 30, 311. This paper uses

the entity-relationship model as a framework from which the three existing data

models may be derived. The reader may view the entity-relationship model as a

generalization or extension of existing models.

This paper is organized into three parts (Sections 2-4). Section 2 introduces

the entity-relationship model using a framework of multilevel views of data.

Section 3 describes the semantic information in the model and its implications for

data description and data manipulation. A special diagrammatric technique, the

entity-relationship diagram, is introduced as a tool for database design. Section 4

analyzes the network model, the relational model, and the entity set model, and

describes how they may be derived from the entity-relationship model.

2. THE ENTITY-RELATIONSHIP MODEL

2.1 Multilevel Views of Doto

In the study of a data model, we should identify the levels of logical views of data

with which the model is concerned. Extending the framework developed in [lS, 251,

we can identify four levels of views of data (Figure 1) :

(1) Information concerning entities and relat,ionships which exist in our minds.

(2) Information struct,ure-organization of information in which entities and

relationships are represented by data.

(3) Access-path-independent data structure-the data structures which are not

involved with search schemes, indexing schemes, etc.

(4) Access-path-dependent data st.ructure.

In the following sections, we shall develop the entity-relationship model step by

step for the first, two levels. As we shall see later in the paper, the network model,

as currently implemented, is mainly concerned with level 4; the relational model is

mainly concerned with levels 3 and 2; the entity set model is mainly concerned

with levels 1 and 2.

2.2 Information Concerning Entities and Relationships (Level 1)

At this level we consider entities and relationships. An entity is a “thing” which

can be distinctly identified. A specific person, company, or event is an example of

an entity. A

relationship

is an association among entities. For instance, “father-son”

is a relationship between two CLperson” entities.’

IIt. is possible that some people may view something (e.g. marriage) as an entity while other

people may view it as a relationship. We think that this is a decision which has to be made by

the enterprise administrator [27]. He should define what are entit,ies and what are relationships

so that the distinction is suitable for his environment.

ACM Transactions on Database Systems, Vol. 1, No. 1. March 1976.

The Entity-Relationship Model * 1 1

LEVELS OF LOGICAL VIEWS

MODELS

ENTITY-RELATIONSHIP NETWORK RELATIONAL ENTITY-SET

LEVEL I

INFORMATION CONCERNING

ENTITIES AND

RELATIONSHIPS

LEVEL 2

INFORMATION STRUCTURE

LEVEL 3

ACCESS-PATH-

INDEPENDENT

DATA STRUCTURE

LEVEL 4

ACCESS-PATH-

DEPENDENT

DATA STRUCTURE

ENTITIES

ENTITY SETS

RELATIONSHIPS ENTITY SETS

RELATIONSHIPS ROLES

RELATIONSHIP SETS ATTRIBUTES

ATTRIBUTES

VALUES

VALUE SETS

ROLES I

/t----\

ENTITY/RELATIONSHIP/ 1

3NF

- -*ENTITY

RELATION f--- SIMILAR+RELATIONS DESCRIPTION

I 4

SETS

ENTITY-RELATIONSHIP

DIAGRAM

DECOMPOSITION

APPROACH

TABLE

RELATIONS

(TABLES)

RECORDS

;;I)-STRUCTURE-

DATA-STRUCTURE-

DIAGRAM

Fig. 1. Analysis of data models using mult.iple levels of logical views

The database of an enterprise contains relevant information concerning entities

and relationships in which the enterprise is interested. A complete description of

an entity or relationship may not be recorded in the database of an enterprise.

It is impossible (and, perhaps, unnecessary) to record every potentially available

piece of information about ent,ities and relationships. From now on, we shall

consider only the entities and relationships (and the information concerning them)

which are to enter into the design of a database.

2.2.1 Entity and Entity Set. Let e denote an entity which exists in our minds.

Entities are classified into different entity sets such as EMPLOYEE, PROJECT,

and DEPARTMENT. There is a predicate associated with each entity set to test

whether an entity belongs to it, For example, if we know

entity is in the entity

set EMPLOYEE, then we know that it has the properties common to the other

entities in the entity set EMPLOYEE. Among these properties is the afore-

mentioned test predicate. Let Ri denote entity sets. Note that entity sets may not

be mutually disjoint. For example, an entity which belongs to the entity set MALE-

PERSON also belongs to the entity set PERSON. In this case, MALE-PERSON

is a subset of PERSON.

2.2.2 Relationship, Role, and Relationship Set. Consider associations among

entities. A relationship set,

Ri,

is a mathematical relation [5] among n entities,

ACM Transactions on Dstabase Systems, Vol. 1, No. 1, March 1976.

12 ’

P. P.-S. Chen

each taken from an entity set:

(Gel,

e2,

. .

. , e,] 1 el E El, e2 E E2, . . . , e, E En},

and each tuple of entities, [el, et, , . .

, e,],

is a

relationship.

Note that the Et in the

above definition may not be distinct. For example, a “marriage” is a relationship

between two entities in the entity set PERSON.

The role of an entity in a relationship is the function that it performs in the

relationship. LLHusband” and “wife” are roles. The ordering of entities in the

definition of relationship (note that square brackets were used) can be dropped if

roles of entities in the relationship are explicitly stated as follows: (rr/er, rs/e2, . . . ,

r,/e,) , where ri is the role of ei in the relationship.

2.2.3 Attribute, Value, and Value Set. The information about an entity or a

relationship is obtained by observation or measurement, and is expressed by a set

of attribute-value pairs. “3”, “red”, “Peter”, and “Johnson” are values. Values

are classified into different

va2ue sets,

such as FEET, COLOR, FIRST-NAME,

and LAST-NAME. There is a predicate associated with each value set to test

whether a value belongs to it. A value in a value set may be equivalent to another

value in a different value set. For example,

“12” in value set INCH is equivalent

to “1” in value set FEET.

attribute

can be formally defined as a function which maps from an entity

set or a relationship set into a value set or a Cartesian product of value sets:

f: Ei or Ri + Vi or Vi, X Vi, X *-a X Vi,.

Figure 2 illustrates some attributes defined on entity set PERSON. The attribute

AGE maps into value set NO-OF-YEARS. An attribute can map into a Cartesian

product of value sets. For example, the attribute NAME maps into value sets

FIRST-NAME, and LAST-NAME. Note that more than one attribute may map

from the same entity set into the same value set (or same group of value sets).

For example, NAME and ALTERNATIVE-NAME map from the entity set

EMPLOYEE into value sets FIRST-NAME and LAST-NAME. Therefore, attri-

bute and value set are different concepts although they may have the same name

in some cases (for example, EMPLOYEE-NO maps from EMPLOYEE to value

set EMPLOYEE-NO). This distinction is not clear in the network model and in

many existing data management systems. Also note that an attribute is defined as

a function. Therefore, it maps a given entity to a single value (or a single tuple of

values in the case of a Cartesian product of value sets).

Note that relationships also have attributes. Consider the relationship set

PROJECT-WORKER (Figure 3). The attribute PERCENTAGE-OF-TIME,

which is the portion of time a particular employee is committed to a particular

project, is an attribute defined on the relationship set PROJECT-WORKER. It

is neither an attribute of EMPLOYEE nor an attribute of PROJECT, since its

meaning depends on both the employee and project involved. The concept of

attribute of relationship is important in understanding the semantics of data and

in determining the functional dependencies among data.

2.2.4 Conceptual Information Structure. We are now concerned with how to

organize the information associated with entities and relationships. The method

proposed in this paper is to separate the information about entities from the infor-

ACM Tranaaotions on Database Systems. Vol. 1, No. 1, Marah 1076.

剩余27页未读，继续阅读

评论收藏

内容反馈

seenangels

粉丝: 2
资源: 1

数据库相关基础介绍（经典）

评论0

最新资源

数据库相关基础介绍（经典）

评论0

VBNET数据库编程基础教程.doc

数据库系统基础知识.pdf

JSP+Oracle数据库组建动态网站经典实例

数据库开发经典实例

数据库应用技术基础内容课件

数据库基础教程

数据库技术基础及应用-3

数据库基础知识 苗雪兰

《数据库系统基础教程》第3版[DJVU]

083_《Delphi数据库开发经典实例精解》(1/2)

java开发面试常见的数据库基础相关的技术点介绍以及面试问题解答

数据库系统基础教程A

数据库基础知识.zip

数据库系统基础知识.doc

数据库原理新技术介绍

MySQL学习笔记1-数据库基础.md

下载sqlncli_x64.msi.zip

2023年更新的最新版全国五级行政区域数据库表以及sql文件

虹软3.0人脸识别客户端（追踪，活体检测，人脸特征存储，人脸识别，人脸注册，人脸匹配）可离线断网部署 5000个免费key/年

数据库系统概念（原书第七版）课后作业题

年度变更举证DB照片查看工具

DB Browser for SQLite 数据库查看工具

Navicat15安装包和安装教程.zip

JAVA课程设计，学生管理系统，设计SQL server数据库操作

i2 Analyst's Notebook for free免狗

基于SpringBoot+Vue实现增删改查和分页查询DEMO（源码+数据库)

mongodb-linux-x86_64-rhel70-4.4.13安装包和conf配置文件

关于MySQL的语句实验一

最新资源

数据库基础知识苗雪兰