Hamish Burke | 2025-03-12

Related to: #databases

Entity-Relationship Data model

• related to q4 in assignment

Entities

• Basic objects that can be identified in the target of the database
• Eg:
  • STUDENT

Attributes

• Properties that describe entities
• Eg entity STUDENT has
  • {StudID,Name,DoB,Email}, primary key {StudID}

Relationships

• Associations between entities, or objects that are derived from entities
• Same as Relation schema in Relational Data Model
• Have entities as their Components
• Also have attributes as properties that describe them
• Drawing
  • Entity is rectangle
  • Relationship types are diamond
  • Arrow going from relationship to entity, means that entity is a component of that relationship
    • Arrow with circle on it means its a key component
• Eg Relationship type ENROL
  • With component set STUDENT,COURSE
  • Attribute set CalYear,Semester,Points
  • Primary key STUDENT,COURSE,CalYear

Recursive Relationship types and roles

• Contains the same entity type more than once as a component
• Eg model a relationship between a child and a parent
  • Two components, CHILD and PARENT

• Entity type and relationship types together form a Entity-Relationship (ER) schema

ER Schema

• A finite set $S$ of entity and relationship types
  • That for every relationship type $R$ in $S$ all its components belong to $S$, too
• Instance of schema = $S^t$