Entity-Relationship Diagrams Conceptual Modeling Entity-Relationship Diagrams

E-R Model Constructs Entities Relationships Attributes

E-R Model Notation Entity Attribute One-to-many relationship Many-to-many relationship

Example of Relationship Type COMPLETES EMPLOYEE COURSE 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 ASSIGNMENT EMPLOYEE PARKING PLACE 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 MEMBERSHIP DEPARTMENT EMPLOYEE A particular department consists of many employees A particular employee belongs to one department

Example of Binary Many-to-Many Relationship MEMBERSHIP EMPLOYEE COMMITTEE 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 MARRIAGE PERSON A particular person may be married to only one other person

Example of Unary One-to_Many Relationship SUPERVISORY EMPLOYEE A particular employee may supervise many employees A particular employee has only one supervisor

Example of Unary Many-to_Many Relationship BILL_OF_MATERIAL PART 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 SKILL EMPLOYEE PROJECT SKILL ASSIGNMENT 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 PART VENDOR WAREHOUSE 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

Example of Ternary Many-Many-One Relationship SEMESTER INSTRUCTOR COURSE TEACHES 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 BELONGS_TO EMPLOYEE DEPARTMENT Belongs EMPLOYEE To Consists_of

Example of Composite Attribute