A2 SSD — Entity Relationship Models

ER Modelling

Entity Relationship (ER) modelling is regarded as a high level, top-down data modelling process which is useful for complex databases.

It starts by looking at the strategy of the organisation and identifies the entities and relationships of interest to the organisation.

May be used for any type of relational database and even for hierarchical or network databases.

ER Models are independent of the type of database being used, or programming languages or hardware.

Terminology

Entity(Type) — A set of objects or concepts (persons, places, things, events) about which data is to be recorded and maintained, relevant to the organisation.

Each entity is uniquely identified. A rectangle labelled with a singular noun.

Relationship(Type) — A meaningful association between two entities. Relationships are labelled with verbs and the relationship between the entities read as a simple sentence.

Cardinality of relationship may be one-to-one, one-to-many, many-to-many.

Cardinality — Shows the maximum number of entities that can be involved in a relationship.