Conceptual Modelling : Entity-Relationship Diagram

Conceptual Modelling : Entity-Relationship Diagram

1. Entity

   • noun
   • CAPITAL LETTERS
   • rectangle shape

2. Relationship

   • verbs
   • diamond shape
   • interaction of entites

3. Attributes

   • features of entites
   • noun
   • oval shape
   • Types
     • Simple
       • Can't subdivide
     • Composite
       • Can subdivide
     • Single
       • Can be composite
     • Multivalued
       • shown by double line connecting the attribute to the entity.
     • Primary key
       • uniquely identifies an entity
     • Foreign key
       • link betwen two entities

4. Cardinality

   • min & max number of instances of Entity B that can be associated with Entity A

5. Connectivity

   • 1:1
   • 1:M
   • M:N
     • must be avoided
     • break it into 1:M