Chapter 4 Entity Relationship ER Modeling BIS 3635

Chapter # 4 Entity Relationship (ER) Modeling BIS 3635 - Database Systems School of Management, Business Information Systems, Assumption University A. Thanop Somprasong

Objectives l In this chapter, you will learn: l The main characteristics of entity relationship components l How relationships between entities are defined, refined, and incorporated into the database design process l How ERD components affect database design and implementation l That real-world database design often requires the reconciliation of conflicting goals

Basic Vocabulary for Relational Database Model E-R Model Programmer General User Relation Entity File Table Attribute Field Column Tuple Entity Member Record Row

Entity Relationship (ER) Modeling l l l ER model forms the basis of an ER diagram ERD represents conceptual database as viewed by end user ERDs depict database’s main components: l Entities l Attributes l Relationships

Entities … l l Refers to entity set and not to single entity occurrence Corresponds to table and not to row in relational environment In Chen and Crow’s Foot models, entity represented by rectangle with entity’s name Entity name, a noun, written in capital letters

Attributes … l l l Characteristics of entities Chen notation: attributes represented by ovals connected to entity rectangle with a line l Each oval contains the name of attribute it represents Crow’s Foot notation: attributes written in attribute box below entity rectangle

Attributes (2) …

Attributes (3) … l l l Required attribute: must have a value Optional attribute: may be left empty Domain: set of possible values for an attribute l Attributes may share a domain Identifiers: one or more attributes that uniquely identify each entity instance Composite identifier: primary key composed of more than one attribute

Attributes (4) …

Attributes (5) … l l Composite attribute can be subdivided Simple attribute cannot be subdivided Single-value attribute can have only a single value Multivalued attributes can have many values

Attributes (6) …

Attributes (7) … l l M: N relationships and multivalued attributes should not be implemented l Create several new attributes for each of the original multivalued attributes components l Create new entity composed of original multivalued attributes components Derived attribute: value may be calculated from other attributes l Need not be physically stored within database

Attributes (8) …

Relationships … l l l Association between entities Participants are entities that participate in a relationship Relationships between entities always operate in both directions Relationship can be classified as 1: M Relationship classification is difficult to establish if only one side of the relationship is known

Connectivity and Cardinality l l l Connectivity l Describes the relationship classification Cardinality l Expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity Established by very concise statements known as business rules

Connectivity and Cardinality (2)

Existence Dependence l l Existence dependence l Entity exists in database only when it is associated with another related entity occurrence Existence independence l Entity can exist apart from one or more related entities l Sometimes such an entity is referred to as a strong or regular entity

Relationship Strength l l Weak (non-identifying) relationships l Exists if PK of related entity does not contain PK component of parent entity Strong (identifying) relationships l Exists when PK of related entity contains PK component of parent entity

Relationship Strength (2)

Relationship Strength (3)

Weak Entities l l Weak entity meets two conditions l Existence-dependent l Primary key partially or totally derived from parent entity in relationship Database designer determines whether an entity is weak based on business rules

Weak Entities (2)

Weak Entities (3)

Relationship Participation l l Optional participation l One entity occurrence does not require corresponding entity occurrence in particular relationship Mandatory participation l One entity occurrence requires corresponding entity occurrence in particular relationship

Relationship Participation (2)

Relationship Participation (3)

Relationship Degree l Indicates number of entities or participants associated with a relationship l Unary relationship l Association is maintained within single entity Binary relationship l Two entities are associated Ternary relationship l Three entities are associated l l

Relationship Degree (2)

Relationship Degree (3)

Recursive Relationships l Relationship can exist between occurrences of the same entity set l Naturally found within unary relationship

Recursive Relationships (2)

Recursive Relationships (3)

Associative (Composite) Entities l l Also known as bridge entities Used to implement M: N relationships Composed of primary keys of each of the entities to be connected May also contain additional attributes that play no role in connective process

Associative (Composite) Entities (2)

Associative (Composite) Entities (3)

Developing an ER Diagram l Database design is an iterative process l Create detailed narrative of organization’s description of operations l Identify business rules based on description of operations l Identify main entities and relationships from business rules l Develop initial ERD l Identify attributes and primary keys that adequately describe entities l Revise and review ERD

Developing an ER Diagram (2)

Developing an ER Diagram (3)

Developing an ER Diagram (4)

Developing an ER Diagram (5)

Developing an ER Diagram (6)

Developing an ER Diagram (7)

Developing an ER Diagram (8)

Developing an ER Diagram (9)

Developing an ER Diagram (10)

Developing an ER Diagram (11)

Database Design Challenges: Conflicting Goals l l Database designers must make design compromises l Conflicting goals: design standards, processing speed, information requirements Important to meet logical requirements and design conventions Design of little value unless it delivers all specified query and reporting requirements Some design and implementation problems do not yield “clean” solutions

Database Design Challenges: Conflicting Goals (2)

BIS 3635 : ASSIGNMENT # 1 l Two questions 1. Compare and contrast between E-R Diagram and UML Class Diagram. (Show the example between each of them) (2 Marks) 2. Give an example of the three types of relationships (Draw in Crow’s Foot E-R Diagram only !) (2 examples per each) (3 Marks) l Submission Date: July 13, 2009 (Late submission will not be accepted) Individual Assignments Submit in form of hard-copy format + hand writing with ABAC papers only ! Total : 5 Marks l l l

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