Entity-Relationship Modeling Part 1

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Entity-Relationship Modelling –Part 1

In this chapter, you will learn:

• ER Diagrams – Crow Feet & Chen Notations

• The main characteristics of entity relationship components

– Entity

– Attributes

• Primary keys

• Composite primary keys

• Composite and simple attributes

• Single-valued attributes

• Multivalued attributes

– Relationships

• 1:M relationship

• 1:1 relationship

• M:N relationship

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The Entity Relationship (ER) Model

• ER model forms the basis of an ER diagrams (Crow Feet Notation & Chen Notation)

• ERD represents conceptual database as viewed by end user

• ERDs depict database’s main components:

– Entities

– Attributes

– Relationships

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Entities

• Refers to entity set and not to single entity occurrence

• Corresponds to table and not to row in relational environment

• In both Chen and Crow’s Foot models, entity is represented by rectangle containing entity’s name

• Entity name, a noun, is usually written in capital letters

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Attributes

• Characteristics of entities

• In Chen model, attributes are represented by ovals and are connected to entity rectangle with a line

• Each oval contains the name of attribute it represents

• In Crow’s Foot model, attributes are written in attribute box below entity rectangle

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Composite and Simple Attributes

• Composite attribute (NOT composite key) can be subdivided to yield additional attributes.

– an ADDRESS attribute can be subdivided into city,

state and postcode

– NAME attribute can be subdivided into first_Name and Last_Name

• Simple attribute cannot be subdivided.

Ex. AGE, MARITAL STATUS

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Single-Valued Attributes

• Single-value attribute can have only a single value

Ex. CAR_MODEL_NO

• Not necessarily a simple attribute

Ex. IC_NUM => 780417-08-5284

is a composite attribute because it can be subdivided into birthdate (780417), place of birth (08), serial number (5284)

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Multivalued Attributes

Multivalued attributes can have many values

Ex. A car color may be subdivided into many colors (colors for the roof, body and trim)

Chen Model, the multivalued attributes are shown by double line connecting the attribute to the entity.

Multivalued Attributes (continued)

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Resolving Multivalued Attribute Problems

• Although conceptual model can handle M:N relationships and multivalued attributes, you should not implement them in relational DBMS

– Option 1: Within original entity, create several new attributes, one for each of the original multivalued attribute’s components

• Can lead to major structural problems in table

– Option 2: Create new entity composed of original multivalued attribute’s components

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Resolving Multivalued Attribute Problems (continued)

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Option 1: Within original entity, create several new attributes

Resolving Multivalued Attribute Problems (continued)

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Option 2: Create new entity composed of original multivalued attribute’s components

Derived Attributes

• Attribute whose value may be calculated (derived) from other attributes.

Ex. EMP_AGE can be derived by computing the difference between the current date and the EMP_DOB

• Need not be physically stored within database.

• Can be derived by using an algorithm.

• Chen Model, the derived attribute is connected to the entity using dashed line.

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Derived Attributes (continued)

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Advantages and Disadvantages of Storing Derived Attributes

Relationships

• Association between entities

• Participants are entities that participate in a relationship

• Relationships between entities always operate in both directions in order to classified whether the relationship is 1:1, 1:M or M;N

• Relationship classification is difficult to establish if only one side of the relationship is known

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Relationships within the Relational Database

• 1:M relationship

– Relational modeling ideal

– Should be the norm in any relational database design

• 1:1 relationship

– Should be rare in any relational database design

• M:N relationships

– Cannot be implemented as such in the relational model

– M:N relationships can be changed into two 1:M relationships

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The 1:M Relationship (continued)

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The 1:M Relationship (continued)

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The 1:1 Relationship

• One entity can be related to only one other entity, and vice versa

• Sometimes means that entity components were not defined properly

• As rare as 1:1 relationships should be, certain conditions absolutely require their use

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The 1:1 Relationship (continued)

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The M:N Relationship• Can be implemented by breaking it up to produce a set of

1:M relationships

• Can avoid problems inherent to M:N relationship by creating a composite entity or bridge entity

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The M:N Relationship (continued)

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The M:N Relationship (continued)

• Implementation of a composite entity

• Yields required M:N to 1:M conversion

• Composite entity table must contain at least the primary keys of original tables

– Also known as bridge entities

– Composed of primary keys of each of the entities to be connected

– May also contain additional attributes that play no role in connective process

• Linking table contains multiple occurrences of the foreign key values

• Additional attributes may be assigned as needed22

The M:N Relationship (continued)

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Changing the M:N relationship to two 1:M relationships

Bridge entity / composite entity

The M:N Relationship (continued)

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Connectivity and Cardinality

• Connectivity

– Used to describe the relationship classification; 1:1. 1:M, M:N

• Cardinality

– Expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity

– In the ERD, it is indicated by placing the appropriate numbers beside the entity using the format (x,y)

• Established by very concise statements known as business rules

– Ex. A professor can only teach up to 4 classes

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Connectivity and Cardinality (continued)

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Existence Dependence/ Existence Independence

• Existence dependence

– Exist in database only when it is associated with another related entity occurrence

– Ex. EMPLOYEE has DEPENDANT. DEPENDANT can’t exist apart from EMPLOYEE

• Existence independence

– Entity can exist apart from one or more related entities

– Sometimes refers to such an entity as strong or regular entity

– Ex. PART is supplied by VENDOR. Some parts are not supplied by vendor because they’re produced in-house

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Relationship Strength

• Weak (non-identifying) relationships

– Exists if PK of related entity does not contain PK component of parent entity

• Strong (Identifying) Relationships

– Exists when PK of related entity contains PK component of parent entity

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Weak Entities

• Weak entity meets two conditions

– Existence-dependent

• Cannot exist without entity with which it has a relationship

– Has primary key that is partially or totally derived from parent entity in relationship

• Database designer usually determines whether an entity can be described as weak based on business rules

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Weak Entities (continued)

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Relationship Participation

• Optional participation

– One entity occurrence does not require corresponding entity occurrence in particular relationship

• Mandatory participation

– One entity occurrence requires corresponding entity occurrence in particular relationship

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Relationship Participation (continued)

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Relationship Participation (continued)

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Relationship Degree

• Indicates number of entities or participants associated with a relationship

• Unary relationship– Association is maintained within single entity

• Binary relationship – Two entities are associated

• Ternary relationship – Three entities are associated

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Relationship Degree (continued)

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The implementation of ternary Relationship

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Recursive Relationships

• Relationship type where same entity type participates more than once in different roles

• Relationship can exist between occurrences of the same entity set

• Naturally found within unary relationship

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Recursive Relationships

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Recursive Relationships (continued)

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Recursive Relationships (continued)

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Summary

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Now you should be able to explain:

ER Diagrams – Crow Feet & Chen Notations

The main characteristics of entity relationship components

– Entity

– Attributes

– Relationships - 1:M relationship , 1:1 relationship and M:N

relationship

Strong and weak identfying relationship

Relationship participation

Relationship degree

Recursive relationship

References

• Database systems: design, implementation, and management, by Peter Rob, Carlos Coronel

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