Chapter 7 Data Modeling Using the EntityRelationship (ER)

Chapter 7 Data Modeling Using the EntityRelationship (ER) Model Copyright 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Chapter 7 Outline Using High-Level Conceptual Data Models for Database Design A Sample Database Application

Entity Types, Entity Sets, Attributes, and Keys Relationship Types, Relationship Sets, Roles, and Structural Constraints Weak Entity Types Copyright 2011 Ramez Elmasri and Shamkant Navathe Chapter 7 Outline (contd.) Refining the ER Design for the COMPANY Database ER Diagrams, Naming Conventions, and Design Issues Example of Other Notation: UML Class Diagrams

Relationship Types of Degree Higher than Two Copyright 2011 Ramez Elmasri and Shamkant Navathe Data Modeling Using the Entity-Relationship (ER) Model Entity-Relationship (ER) model Popular high-level conceptual data model ER diagrams

Diagrammatic notation associated with the ER model Unified Modeling Language (UML) Copyright 2011 Ramez Elmasri and Shamkant Navathe Using High-Level Conceptual Data Models for Database Design Requirements collection and analysis

Database designers interview prospective database users to understand and document data requirements Result: data requirements Functional requirements of the application Copyright 2011 Ramez Elmasri and Shamkant Navathe Using High-Level Conceptual Data Models (contd.) Conceptual schema

Conceptual design Description of data requirements Includes detailed descriptions of the entity types, relationships, and constraints Transformed from high-level data model into implementation data model Copyright 2011 Ramez Elmasri and Shamkant Navathe Using High-Level Conceptual

Data Models (contd.) Logical design or data model mapping Result is a database schema in implementation data model of DBMS Physical design phase Internal storage structures, file organizations, indexes, access paths, and physical design parameters for the database files specified

Copyright 2011 Ramez Elmasri and Shamkant Navathe A Sample Database Application COMPANY Employees, departments, and projects Company is organized into departments Department controls a number of projects

Employee: store each employees name, Social Security number, address, salary, sex (gender), and birth date Keep track of the dependents of each employee Copyright 2011 Ramez Elmasri and Shamkant Navathe Copyright 2011 Ramez Elmasri and Shamkant Navathe Entity Types, Entity Sets, Attributes, and Keys ER model describes data as:

Entities Relationships Attributes Copyright 2011 Ramez Elmasri and Shamkant Navathe Entities and Attributes Entity Thing in real world with independent existence

Attributes Particular properties that describe entity Types of attributes: Composite versus simple (atomic) attributes

Single-valued versus multivalued attributes Stored versus derived attributes NULL values Complex attributes Copyright 2011 Ramez Elmasri and Shamkant Navathe Entities and Attributes (contd.) Copyright 2011 Ramez Elmasri and Shamkant Navathe Entity Types, Entity Sets, Keys, and Value Sets Entity type

Collection (or set) of entities that have the same attributes Copyright 2011 Ramez Elmasri and Shamkant Navathe Entity Types, Entity Sets, Keys, and Value Sets (contd.) Key or uniqueness constraint Attributes whose values are distinct for each

individual entity in entity set Key attribute Uniqueness property must hold for every entity set of the entity type Value sets (or domain of values) Specifies set of values that may be assigned to that attribute for each individual entity Copyright 2011 Ramez Elmasri and Shamkant Navathe Initial Conceptual Design of the

COMPANY Database Copyright 2011 Ramez Elmasri and Shamkant Navathe Relationship Types, Relationship Sets, Roles, and Structural Constraints Relationship When an attribute of one entity type refers to another entity type Represent references as relationships not

attributes Copyright 2011 Ramez Elmasri and Shamkant Navathe Relationship Types, Sets, and Instances Relationship type R among n entity types E1, E2, ..., En Defines a set of associations among entities from these entity types Relationship instances ri

Each ri associates n individual entities (e1, e2, ..., en) Each entity ej in ri is a member of entity set Ej Copyright 2011 Ramez Elmasri and Shamkant Navathe Relationship Degree Degree of a relationship type

Number of participating entity types Binary, ternary Relationships as attributes Think of a binary relationship type in terms of attributes Copyright 2011 Ramez Elmasri and Shamkant Navathe Copyright 2011 Ramez Elmasri and Shamkant Navathe

Role Names and Recursive Relationships Role names and recursive relationships Role name signifies role that a participating entity plays in each relationship instance Recursive relationships Same entity type participates more than once

in a relationship type in different roles Must specify role name Copyright 2011 Ramez Elmasri and Shamkant Navathe Copyright 2011 Ramez Elmasri and Shamkant Navathe Constraints on Binary Relationship Types Cardinality ratio for a binary relationship Specifies maximum number of relationship instances that entity can participate in

Participation constraint Specifies whether existence of entity depends on its being related to another entity Types: total and partial Copyright 2011 Ramez Elmasri and Shamkant Navathe Attributes of Relationship Types Attributes of 1:1 or 1:N relationship types can be migrated to one entity type

For a 1:N relationship type Relationship attribute can be migrated only to entity type on N-side of relationship For M:N relationship types Some attributes may be determined by combination of participating entities Must be specified as relationship attributes

Copyright 2011 Ramez Elmasri and Shamkant Navathe Weak Entity Types Do not have key attributes of their own Identified by being related to specific entities from another entity type Identifying relationship Relates a weak entity type to its owner

Always has a total participation constraint Copyright 2011 Ramez Elmasri and Shamkant Navathe Refining the ER Design for the COMPANY Database Change attributes that represent relationships into relationship types Determine cardinality ratio and participation constraint of each relationship type Copyright 2011 Ramez Elmasri and Shamkant Navathe ER Diagrams, Naming

Conventions, and Design Issues Copyright 2011 Ramez Elmasri and Shamkant Navathe Proper Naming of Schema Constructs Choose names that convey meanings attached to different constructs in schema Nouns give rise to entity type names Verbs indicate names of relationship types Choose binary relationship names to make ER diagram readable from left to right and from top to bottom

Copyright 2011 Ramez Elmasri and Shamkant Navathe Design Choices for ER Conceptual Design Model concept first as an attribute Refined into a relationship if attribute is a reference to another entity type Attribute that exists in several entity types may be elevated to an independent entity type

Can also be applied in the inverse Copyright 2011 Ramez Elmasri and Shamkant Navathe Alternative Notations for ER Diagrams Specify structural constraints on relationships Replaces cardinality ratio (1:1, 1:N, M:N) and

single/double line notation for participation constraints Associate a pair of integer numbers (min, max) with each participation of an entity type E in a relationship type R, where 0 min max and max 1 Copyright 2011 Ramez Elmasri and Shamkant Navathe Copyright 2011 Ramez Elmasri and Shamkant Navathe Example of Other Notation: UML Class Diagrams UML methodology

Used extensively in software design Many types of diagrams for various software design purposes UML class diagrams Entity in ER corresponds to an object in UML Copyright 2011 Ramez Elmasri and Shamkant Navathe

Copyright 2011 Ramez Elmasri and Shamkant Navathe Example of Other Notation: UML Class Diagrams (contd.) Class includes three sections: Top section gives the class name Middle section includes the attributes; Last section includes operations that can be applied to individual objects

Copyright 2011 Ramez Elmasri and Shamkant Navathe Example of Other Notation: UML Class Diagrams (contd.) Associations: relationship types Relationship instances: links Binary association Represented as a line connecting participating classes May optionally have a name

Link attribute Placed in a box connected to the associations line by a dashed line Copyright 2011 Ramez Elmasri and Shamkant Navathe Example of Other Notation: UML Class Diagrams (contd.) Multiplicities: min..max, asterisk (*) indicates no maximum limit on participation Types of relationships: association and aggregation

Distinguish between unidirectional and bidirectional associations Model weak entities using qualified association Copyright 2011 Ramez Elmasri and Shamkant Navathe Relationship Types of Degree Higher than Two Degree of a relationship type Number of participating entity types Binary

Relationship type of degree two Ternary Relationship type of degree three Copyright 2011 Ramez Elmasri and Shamkant Navathe Choosing between Binary and Ternary (or Higher-Degree) Relationships

Some database design tools permit only binary relationships Ternary relationship must be represented as a weak entity type No partial key and three identifying relationships Represent ternary relationship as a regular entity type

By introducing an artificial or surrogate key Copyright 2011 Ramez Elmasri and Shamkant Navathe Copyright 2011 Ramez Elmasri and Shamkant Navathe Constraints on Ternary (or Higher-Degree) Relationships Notations for specifying structural constraints on n-ary relationships Should both be used if it is important to fully

specify structural constraints Copyright 2011 Ramez Elmasri and Shamkant Navathe Summary Basic ER model concepts of entities and their attributes Different types of attributes Structural constraints on relationships ER diagrams represent E-R schemas

UML class diagrams relate to ER modeling concepts Copyright 2011 Ramez Elmasri and Shamkant Navathe