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Entity-Relationship (ER) Modeling

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Presentation on theme: "Entity-Relationship (ER) Modeling"— Presentation transcript:

1 Entity-Relationship (ER) Modeling

2 Database Design Process
Application 1 Application 2 Application 3 Application 4 External Model External Model External Model External Model Application 1 Conceptual requirements Application 2 Conceptual requirements Conceptual Model Logical Model Internal Model Application 3 Conceptual requirements Application 4 Conceptual requirements

3 Stages in Database Design
Requirements formulation and analysis Conceptual Design -- Conceptual Model Implementation Design -- Logical Model Physical Design --Physical Model

4 Database Design Process
Requirements formulation and analysis Purpose: Identify and describe the data that are used by the organization Results: Metadata identified, Data Dictionary, Conceptual Model-- ER diagram

5 Database Design Process
Requirements Formulation and analysis Systems Analysis Process Examine all of the information sources used in existing applications Identify the characteristics of each data element numeric text date/time etc. Examine the tasks carried out using the information Examine results or reports created using the information

6 Conceptual Modeling Objective: to produce HIGH-LEVEL DATA MODEL GOALS
a complete understanding of the database structure, meaning (semantics), interrelationships, and constraints A stable description of the database contents Usually more expressive and general than data models of individual DBMSs Vehicle of communication among database users, designers, and analysts.

7 Database Design Process
Conceptual Model Merge the collective needs of all applications Determine what Entities are being used Some object about which information is to maintained What are the Attributes of those entities? Properties or characteristics of the entity What attributes uniquely identify the entity What are the Relationships between entities How the entities interact with each other?

8 Database Design Process
Logical Model How is each entity and relationship represented in the Data Model of the DBMS Hierarchic? Network? Relational? Object-Oriented?

9 Database Design Process
Physical ( Internal) Model Choices of index file structure Choices of data storage formats Choices of disk layout

10 Database Design Process
External Model User views of the integrated database Making the old (or updated) applications work with the new database design

11 Data Models: History Relational Model (1980’s)
Provides a conceptually simple model for data as relations (typically considered “tables”) with all data visible.

12 Data Models: History Object Oriented Data Model (1990’s)
Encapsulates data and operations as “Objects” Books (id, title) Publisher Subjects Authors (first, last)

13 Intro. to ER Models Entity/Relationship approach - one of the most well known modeling methods Developed by P.Chen in many variations since then Data modeling is generally considered the most important component of the systems development process

14 Entity Relationship Diagrams
A Simple ERD:  Consider the following situation A customer places an order. The order consists of parts. Entity Relationship Another Relationship Places Contain Customer Orders Parts An Organization about which we wish to maintain information An Association between Entities Another Entity Altogether, a Database

15 ER NOTATION Attribute Entity type Key attribute Weak entity type
Multivalued attribute Relationship type Derived attribute Identifying Relationship type Composite attribute

16 A special entity that is also a relationship
Relationship symbols Entity symbols Attribute symbols A special entity that is also a relationship Relationship degrees specify number of entity types involved Relationship cardinalities specify how many of each entity type is allowed 8

17 Entity An Entity is an object in the real world (or even imaginary worlds) about which we want or need to maintain information Persons (e.g.: customers in a business, employees, authors) Things (e.g.: purchase orders, meetings, parts, companies) Employee

18 Inappropriate entities
Figure 3-4 System output System user Appropriate entities

19 Identifiers (Keys) Identifier (Key) - An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type Simple Key versus Composite Key Candidate Key – an attribute that could be a key…satisfies the requirements for being a key 6

20 Characteristics of Identifiers
Will not change in value Will not be null No intelligent identifiers (e.g. containing locations or people that might change) 7

21 Attributes Attributes are the significant properties or characteristics of an entity that help identify it and provide the information needed to interact with it or use it. (This is the Metadata for the entities.) Employee Last Middle First Name SSN Age Birthdate Projects

22 Figure 3-7 – A composite attribute
An attribute broken into component parts 12

23 Weak Entities Owe existence entirely to another entity Order-line
Contains Order Invoice # Part# Rep# Quantity Invoice#

24 Figure 3-9a – Simple key attribute
The key is underlined 14

25 Figure 3-9b – Composite key attribute
The key is composed of two subparts 15

26 Figure 3-8 – Entity with a multivalued attribute (Skill) and derived attribute (Years_Employed)
What’s wrong with this? Derived from date employed and current date Multivalued: an employee can have more than one skill 13

27 Relationships Relationships are the associations between entities. They can involve one or more entities and belong to particular relationship types

28 Relationships Class Attends Student Part Supplier Project Supplies

29 Relationships Class Attends Student Part Supplier Project Supplies

30 Types of Relationships
Concerned only with cardinality of relationship Truck Assigned Employee Project 1 n m Chen ER notation

31 Other Notations “Crow’s Foot” Truck Employee Project Employee Project
Assigned Employee Project Assigned Employee Project Assigned Employee “Crow’s Foot”

32 Degree of Relationships
Degree of a relationship is the number of entity types that participate in it Unary Relationship Binary Relationship Ternary Relationship 16

33 Degree of relationships – from Figure 3-2
Entities of two different types related to each other One entity related to another of the same entity type Entities of three different types related to each other 8

34 Cardinality of Relationships
One-to-One Each entity in the relationship will have exactly one related entity One-to-Many An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity Many-to-Many Entities on both sides of the relationship can have many related entities on the other side

35 Cardinality Constraints
Cardinality Constraints - the number of instances of one entity that can or must be associated with each instance of another entity Minimum Cardinality If zero, then optional If one or more, then mandatory Maximum Cardinality The maximum number 29

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