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

© 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

© 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

© 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

© 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

© 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

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

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

© 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.

© 2002 by Prentice Hall 10 An Entity Example

© 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)

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

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

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

© 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.

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

© 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

© 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

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

© 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)

© 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

© 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)

© 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

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

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

© 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

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

© 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.

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

© 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.

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

© 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

© 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

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