Monthly Archives

June 2017

Database Management Systems

Single Instance Exception in Entity-Relationship Diagram Design

Entities in the ER (Entity Relationship) model are typically nouns, existing physically, digitally, or conceptually. It is often suggested that entities should only exist (particularly in regards to the design phase) if they will contain more than one instance. For example an Employees entity would conceivably have multiple employees. If it is known that there would only ever be one employee, and that the employee would never change, then logically any information about that employee could be kept in the Application domain, outside of the Database altogether.

The idea behind this concept is that executing queries on data which is static in nature is a resource waste.