01 ER

Foundations

An entity–relationship model (or ER model) describes interrelated things of interest in a specific domain of knowledge. A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that can exist between entities (instances of those entity types).

• Entity: Real-world object distinguishable from other objects. An entity is described using a set of attributes.
• Entity Set: A collection of entities of the same kind (E.g., all employees).
  • All entities in an entity set have the same set of attributes.
  • Each entity set has a key (a set of attributes uniquely identifying an entity).
  • Each attribute has a domain.
• Relationship: Association among two or more entities.
• Relationship Set: Collection of similar relationships.
• ISA Hierarchies: Divide an entity in subentities which share the attributes and relationship of the parent entity.
  • Total: each instance of the parent entity must be part of at least one of the children entities
  • Exclusive: each instance of the parent entity must not be part of more than one of the children entities
  • Partition: A total end exclusive hierarchy

Design Methodology

Three main strategies:

1. Top down
2. Bottom up
3. Inside out