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Entity-Relationship (ER) Modeling. Database Design Process Conceptual Model Logical Model External Model Conceptual requirements Conceptual requirements.

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Presentation on theme: "Entity-Relationship (ER) Modeling. Database Design Process Conceptual Model Logical Model External Model Conceptual requirements Conceptual requirements."— Presentation transcript:

1 Entity-Relationship (ER) Modeling

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

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

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

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

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 System user System output Appropriate entities Figure 3-4

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

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)

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

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

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

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

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

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 Supplies project parts Supplier Project

29 Relationships Class Attends Student Part Supplies project parts Supplier Project

30 Types of Relationships Concerned only with cardinality of relationship Truck Assigned Employee Project Assigned Employee Project Assigned Employee 11 n n 1 m Chen ER notation

31 Other Notations Truck Assigned EmployeeProject Assigned EmployeeProject 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

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

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

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