Entity Relationship Models

Entity

An entity is a real-world object or concept represented in an ERD. For example, in a college database, Student is an entity.

Examples: Teachers, Course, Building, Classroom.

Representation: A rectangle.

Entity set: A collection of similar entities (e.g., all students).

Attributes are properties of an entity. For a Student entity, typical attributes are:

• ID
• Email
• Age
• Address
• Phone number

Representation: Ovals (connected to the entity).

Common attribute types include:

• Key attribute
• Composite attribute
• Multivalued attribute
• Derived attribute

A key attribute uniquely identifies an entity instance. Example: Student roll number or Course code (e.g., IT101).

Representation: An oval with an underline.

A composite attribute consists of multiple sub-attributes. Example: Address may include House number, Street, City, State, Country, Pin code.

Representation: An oval connected to sub-ovals for components.

A multivalued attribute can have multiple values for one entity. Example: a student may have multiple phone numbers.

Representation: A double-lined oval.

A derived attribute is computed from other attributes. Example: Age can be derived from Date of Birth.

Representation: A dashed oval.

A relationship represents an association between entities. Example: Students enroll in Courses.

Representation: A diamond connecting related entities.

Types of relationships:

• Unary (recursive)
• Binary
• n-ary

A relationship set groups similar relationships between entity sets.

A unary (recursive) relationship involves a single entity type related to itself. Example: A Person entity may have a married_to relationship to another Person.

A binary relationship connects two different entity types. Example: Students enrolled in a Course.

An n-ary relationship involves more than two entity types. These are useful for complex associations but often transformed into simpler tables for implementation.

Example (conceptual): entities like Person, Animal, Plant participating in an eats relationship.

Total Participation

All entities in an entity set must participate in the relationship (represented by a double line). Example: University requires every student to enroll in at least one course (policy example).

Partial Participation

Only some entities participate in the relationship (represented by a single line). Example: Some students may choose not to enroll in optional activities.

A weak entity depends on another (strong) entity for its identity and usually lacks a fully unique key on its own. Example: Installment depends on Loan. Weak entities typically have total participation in the identifying relationship.

Representation: Double rectangle (entity) and double diamond (identifying relationship).