An entity relationship diagram is a graphical representation of an organisation's data storage requirements. Entity relationship diagrams are abstractions of the real world which simplify the problem to be solved while retaining its essential features. Entity relationship diagrams are used to: identify the data that must be captured, stored and retrieved in order to support the business activities performed by an organisation; and identify the data required to derive and report on the performance measures that an organization should be monitoring. Entity relationship diagrams have three different components:
These are the people, places, things, events and concepts of interest to an organisation. In short, anything which an organisation needs to store data about. Entities are represented on the diagram by labelled boxes.
Entities represent collections of things. For example, an EMPLOYEE entity might represent a collection of all the employees that work for an organisation. Individual members (employees) of the collection are called occurrences of the EMPLOYEE entity. Because the available space for naming the entity is restricted to the size of the box, Entities should always have detailed descriptions. These detailed descriptions are usually short paragraphs of text describing the entity in more detail but for some important entities, a lengthy description may be required.
Entities are further described by their attributes (sometimes called data elements). These are the smallest units of data that can be described in a meaningful manner. For example, an EMPLOYEE entity may have the following Attributes:
Frequently, a meaningful relationship exists between two different types of entity.
EMPLOYEEs work in a DEPARTMENT LAWYERs advise CLIENTs
EQUIPMENT is allocated to PROJECTs
TRUCK is a type of VEHICLE