Conceptual ModelingEntity-Relationship Diagrams

E-R Model Constructs

• Entities• Relationships • Attributes

E-R Model Notation

Entity

Attribute

Many-to-many relationship

One-to-many relationship

Example of Relationship Type

EMPLOYEE COURSE

COMPLETES

A particular employee may complete many courses

A particular course may be completed by many employees

Cardinality Constraints

A cardinality constraint specifies the number of instances that can, or must, participate in a relationship.

Degree of a Relationship

Refers to the number of entity types that participate in the relationship.

• Unary (degree 1)• Binary (degree 2)• Ternary (degree 3)

Binary Relationships

Relationship between the instances of two entity types.

Example of Binary One-to-One Relationship

EMPLOYEE PARKING PLACE

ASSIGNMENT

A particular employee is assigned one parking place

A particular parking place may be assigned to only one employee

Example of Binary One-to-Many Relationship

DEPARTMENT EMPLOYEE

MEMBERSHIP

A particular department consists of many employees

A particular employee belongs to one department

Example of Binary Many-to-Many Relationship

EMPLOYEE COMMITTEE

MEMBERSHIP

A particular employee may belong to many committees

A particular committee consists of many employees

Unary Relationship

• A unary relationship, also called a recursive relationship, is a relationship between the instances of a single entity type.

Example of Unary One-to-One Relationship

PERSON

A particular person may be married to only one other person

MARRIAGE

Example of Unary One-to_Many Relationship

EMPLOYEESUPERVISORY

A particular employee may supervise many employees

A particular employee has only one supervisor

Example of Unary Many-to_Many Relationship

PARTBILL_OF_MATERIAL

A particular part may consist of many component parts

A particular part may be the component for many parts

Ternary Relationship

A ternary relationship is a simultaneous relationship among the instances of three entity types.

Example of Ternary Many-Many-Many Relationship

EMPLOYEE PROJECT

SKILL

SKILLASSIGNMENT

A particular employee may use a particular skill on many projects

In a particular project, a particular skill may be used by many employees

A particular employee working on a particular project may use many skills

Another Example of Ternary Many-Many-Many Relationship

VENDOR WAREHOUSE

PART

SUPPLIES

A particular part , supplied by a particular vendor, may be stored in many warehouses

A particular part, stored in a particular warehouse, may be supplied by many vendors

In a particular warehouse, a particular vendor may supply many parts

INSTRUCTOR COURSE

SEMESTER

TEACHES

Example of Ternary Many-Many-One Relationship

In a particular semester, a particular instructor may teach many courses

A particular instructor may teach a particular course in many semesters

In a particular semester, a particular course is taught by only one instructor

Comparing Notation

DEPARTMENTEMPLOYEEBelongsTo

DEPARTMENTEMPLOYEE

Belongs_to

Consists_of

DEPARTMENTEMPLOYEE

BELONGS_TO

Example of Composite Attribute