Data Modeling with the Entity-Relationship Model

Entity-Relationship (E-R) Model

First described in a paper published by Peter Chen in 1976. He set out the basic elements of the model. Sub-types were added to create the extended E-R model.

IS-A Relationships

The relationships that connect supertypes and subtypes

Optional-To-Mandatory (O-M) Relationship

The combination of a circle and a hash mark indicates

One-To-One (1:1) Relationship

an entity instance of one type is related to at most one entity instance of the other type

Minimum Cardinality

the min number of entity instances that must participate in a relationship instance

Discriminator

An attribute that determines which subtype is appropriate

When discussing 1:N relationships

the terms parent and child are sometimes used

Relationship Classes

Associations among entity classes, and relationship instances are associations among entity instances

Mandatory

Minimums are stated as either zero or one. If one, at least one entity instance must participate in the relationship

Optional

Minimums are stated as either zero or one. If zero, then the participation in the relationship is optional

Crow's Foot Symbol

Shows the many side of a relationship

Many-To-Many (N:M)

EX: the employee instance can be associated with many skill instances

Ternary Relationships

Relationships of degree three

Cardinality

in the E-R model. relationships are classified by their cardinality, a word that means count

Identifying Relationship

Use a solid line to represent the relationship between the ID-Dependent entity and its parent

Data modeling occurs in the

requirements analysis step of the systems development life cycle (SDLC) in the systems analysis and design process

Parent

the entity on the 1 side of the relationship

Child

the entity on the many side of the relationship

Maximum Cardinality

the max number of entity instances that can participate in a relationship instance

Degree

The number of entity classes in a relationship is the degree of the relationship

Inclusive Subtype

a supertype instance can relate to one or more subtypes

Mandatory-To-Optional (M-O) Relationship

...

Integrated Definition 1, Extended (IDEF1X)

A national standard. Incorporated the basic ideas of the E-R model but uses different graphical symbols

Non-identifying Relationship

A relationship drawn with a dashed line is used between strong entities. there are no ID-dependent entities in the relationship

Weak Entity

Defined as any entity whose existence depends on the presence of another entity

Information Engineering (IE) Model

Developed by James Martin in 1990. Uses crow's feet to show the many side of a relationship, this is called the IE Crow's Foot Model

Optional-To-Optional (O-O) Relationship

EX: and employee need not have a computer, and a computer need not be assigned to an employee. -Two small circles

HAS-A-Relationships

Each instance has a relationship to a second entity instance

Mandatory-to-Mandatory (M-M) Relationship

Entities are required on both sides

Relationships

Entities can be associated with one another in relationships

Attributes

Entities have attributes that describe their characteristics EX: EmployeeNumber, Phone, Email

Entity Class

Entities of a given type are groups into an entity class -Usually has many instances of an entity

Identifiers

Entity instances have identifiers, which are attributes that name, or identify, entity instances

Unified Modeling Language (UML)

Object-oriented development methodology adopted the E-R model but introduced its own symbols while putting an object-oriented programming spin on it

Recursive Relationship (Unary Relationship)

Occurs when an entity type has a relationship to itself

Entity Instance

Of an entity class is the occurrence of a particular entity, such as CUSTOMER 12345

Binary Relationships

Relationships of degree two

ID-Dependent Entity

Special type of weak entity. An entity whose identifier includes the identifier of another entity

Association Pattern

Subtly and confusingly similar to an N:M strong relationship

Entity-Relationship (E-R) Diagrams

Such diagrams use diamonds for relationships, rectangles for entities, and connected ellipses for attributes

Composite Identifiers

The identifier of an entity instance consists of one or more of the entity's attributes. Identifiers consist of two or more attributes are called composite identifiers

Owner Entity

The parent entity of a child ID-dependent entity is sometimes referred to as an owner entity. EX: a building is the owner of the apartments within it

Data model

a plan, or blueprint, for a database design - it is a generalized, non-DBMS specific design

One-To-Many (1:N) Relationship

a single instance can be associated with many instances

Exclusive Subtypes

a supertype instance is related to at most one subtype

Strong Entity

an entity that represents something that can exist on its own

Subtype

entity is a special case of another entity called its supertype

E-R model contains

relationships among classes and relationship instances

Entity

something that users want to track