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
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