E/R → Entity-Relationship Model

Q: How should we design tables in our database?

• Tables are not "given"
• "Good" tables may not be easy to come up with

E/R Model: graphical, intuitive and "informal" representation of information on database

• Used to capture what we learn from domain experts/database users
• Not directly implemented by DBMS
• Tools exist to automatically convert E/R model into tables

2 Important topic: Entity sets and relationship sets

Entity Set

• Entity: "thing" or "object" in real world
  • Eg. Book, UCLA, Student
• Entity set: a set of entities(objects). Like a class in OOP
  • Rectangle in ER
  • Consists of "name" and "attributes"
  • Entities with attributes can be thought as "tuples" or records
  • Key: A set of attributes that uniquely identifiers an entity in an entity set
    • Underline in E/R
    • All entity sets in E/R need a key

Relationship Set

• Relationship: "Connection" between entites
• Relationship set: a set of relations of the same kind
  • Diamond in ER
  • Relationships can be thought as "edges" between entities
• Relationships can have attributes
• Not all entities have to participate in a relationship

Cardinality of Relationships