1. © 2002 by Prentice Hall 1 David M. Kroenke Database Processing Eighth Edition Chapter 3 The Entity- Relationship Model

2. © 2002 by Prentice Hall 2 Data Modeling Process of creating a logical representation of the structure of the database The most important task in database development

3. © 2002 by Prentice Hall 3 The Data Model A data model defines and graphically depicts the data structure and relationships among the data A vocabulary and tool-set for creating the user’s data model

4. © 2002 by Prentice Hall 4 Data Modeling Creation Interviewing users Documenting requirements Building a users data model –Using Entity-Relationship Model Building a database prototype A process of inference –Working backwards

5. © 2002 by Prentice Hall 5 Business Rules Statements that define or constrain some aspect of the business Assert business structure Control/influence business behavior Expressed in terms familiar to end users Automated through DBMS software

6. © 2002 by Prentice Hall 6 A Good Business Rule is: Declarative – what, not how Precise – clear, agreed-upon meaning Atomic – one statement Consistent – internally and externally Expressible – structured, natural language Distinct – non-redundant Business-oriented – understood by business people

7. © 2002 by Prentice Hall 7 Common Data Models Entity-Relationship Model Semantic Object Model

8. © 2002 by Prentice Hall 8 Entity-Relationship Model (E-R Model) An Entity-Relationship Model (E-R Model) consists of: –Entities –Attributes –Identifiers –Relationships

9. © 2002 by Prentice Hall 9 An Entity An entity is an object that can be identified in the users’ work environment & that users want to track. Entities of a given type are grouped into entity classes. An entity instance is the representation of a particular entity.

10. © 2002 by Prentice Hall 10 An Entity Example

11. © 2002 by Prentice Hall 11 What Should an Entity Be? SHOULD BE: –An object that will have many instances in the database –An object that will be composed of multiple attributes –An object that we are trying to model SHOULD NOT BE: –A user of the database system –An output of the database system (e.g. a report)

12. © 2002 by Prentice Hall 12 System user System output Inappropriate Entities

13. © 2002 by Prentice Hall 13 Attributes An attribute describes a characteristic of an entity For example –An entity: Employee –Has attributes: EmployeeName Extension DateOfHire

14. © 2002 by Prentice Hall 14 Attributes Classifications of attributes: –Simple versus Composite Attribute –Single-Valued versus Multivalued Attribute –Stored versus Derived Attributes –Identifier Attributes

15. © 2002 by Prentice Hall 15 Identifier An identifier uniquely identifies a row in a table. For an Employee, the SocialSecurityNumber may serve as the Indentifier.

16. © 2002 by Prentice Hall 16 Characteristics of Identifiers Will not change in value Will not be null

17. © 2002 by Prentice Hall 17 Relationships A relationship describes how one or more entities are related with each other. Relationship Classes are associations among entity classes Relationship Instances are associations among entity instances Relationships can have attributes

18. © 2002 by Prentice Hall 18 Degree of Relationship Degree of relationship is number of entity classes in the relationship Binary relationships (Degree 2) are very common Degree 2Degree 3

19. © 2002 by Prentice Hall 19 Relationship Cardinality Entity-Instance Participation in relationships is shown by –maximum cardinality –minimum cardinality

20. © 2002 by Prentice Hall 20 Maximum Cardinality The maximum cardinality indicates/depicts the maximum number of instances involved in a relationship. Alternatives include –1:1 (one-to-one) –1:N (one-to-many) –N:M (many-to-many)

21. © 2002 by Prentice Hall 21 Relationship Examples Showing Maximum Cardinality Alternatives Employee has just 1 Auto, and vice- versa Dorm can have many students, Student can have just 1 dorm Student can be in many clubs, Club can have many students

22. © 2002 by Prentice Hall 22 Minimum Cardinality The minimum cardinality indicates/depicts whether participation in the relationship is mandatory or optional. Alternatives include –0 (optional) –1 (mandatory)

23. © 2002 by Prentice Hall 23 A Relationship Example Showing Minimum and Maximum Cardinality Minimum Cardinality: Dormitory must have at least 1 Student Student not required to be in a Dorm Maximum Cardinality: Dormitory can have many Students Student can be in at most 1 Dorm

24. © 2002 by Prentice Hall 24 A Recursive Relationship A recursive relationship is when an entity has a relationship with itself. Employee Manages 1:N

25. © 2002 by Prentice Hall 25 Showing Attributes in Entity- Relationship Diagrams Note that relationship Dorm-Occupant has an attribute (Rent).

26. © 2002 by Prentice Hall 26 Entity-Relationship Diagram (E-R Diagram) An entity-relationship diagram (E-R Diagram) is a graphical representation of the E-R model using a set of ‘somewhat’ standardized conventions

27. © 2002 by Prentice Hall 27 An Entity-Relationship Diagram (E-R Diagram) Example

28. © 2002 by Prentice Hall 28 Weak Entity A weak entity is an entity whose instance survival depends (logically) on an associated instance in another entity (strong entity). Rounded Corners signify weak entities.

29. © 2002 by Prentice Hall 29 ID-dependent Entity Where the identifier of one entity includes the identifier of another.

30. © 2002 by Prentice Hall 30 Subtype Entities Some entities may have many common attributes and a few unique attributes. The common attributes may be grouped together in a supertype entity and the unique attributes may be grouped together in a subtype entity.

31. © 2002 by Prentice Hall 31 CLIENT with Subtype Entities Indicates sub- type

32. © 2002 by Prentice Hall 32 E-R Diagram Computer Assisted Software Engineering (CASE) Tools Several Computer Assisted Software Engineering (CASE) Tools exist to help create E-R Diagrams and the resulting physical database elements. Products include: –IEW –IEF –DEFT –ER-WIN –Visio

33. © 2002 by Prentice Hall 33 Unified Modeling Language (UML) The Unified Modeling Language (UML) is a set of structures and techniques for modeling and designing object-oriented programs (OOP) and applications. A primary difference between UML & E-R Diagrams is that the UML representation includes information about object constraints and methods

34. © 2002 by Prentice Hall 34 David M. Kroenke Database Processing Eighth Edition Chapter 3 The Entity- Relationship Model