An Entity Relationship Diagram is a graphical representation of an organization'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.

So what exactly are they???

In other words, entity relationship diagrams are graphical representations that illustrate the logical structure of databases.

What are Entity Relationship Diagrams used for???

Entity relationship diagrams have three different components:


These are the people, places, things, events and concepts of interest to an organization. In short, anything which an organization needs to store data about.

Entities are represented on the diagram by labeled boxes.

Entities represent collections of things. For example, an EMPLOYEE entity might represent a collection of all the employees that work for an organization. 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


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.

For example:

There are potentially three types of relationship which can exist between two different entities:

A sample ERD that describes the address book.

Tips on building ERD’s:

Questions that will help you understand better this topic:

Q) What is an ERD?

      An Entity Relationship Diagram is graphical representation illustrating the logical structure of a database.

Q) What are the three main components of an ERD?

      Entities, Attributes and Relationships.

Q) Explain the difference between Entities, Attributes and Relationships.

      Entities are anything which an organization needs to store data about.

      Attributes are the data we collect about the entities.

      Relationships are the associations that exists between two different types of entity.

Q) Which of the following is a type of relationship that can exist between two entities?

      a) One-to-One relationships b) One-to-Many relationships c) Many-to-Many relationships d) None of the above e) All of the above

Answer: e


http://www.umsl.edu/~sauter/analysis/er/er_intro.html (A description of what an ERD is)

http://www.cit.cornell.edu/atc/materials/old/dbdesign/erd.shtml (A different description of what an ERD is)

Elementary Structures in Entity-Relationship Diagrams: A New Metric for Effort Estimation

Geoffrey J. Kennedy , University of Otago, Dunedin, New Zealand

This paper describes a systematic treatment of the entity- relationship diagram resulting in the identification of three elementary data structures. Recognition of these structures is shown to constitute a useful analytical and diagnostic tool for data modeling. An algorithm for counting the number of occurrences of each type of structure in a data model is presented and the possibility is explored of using these values as the basis for a metric for system development effort estimation.

Deductive Entity Relationship Modeling

G.  Di Battista, M.  Lenzerini

An entity relationship oriented model, that includes the notion of class, together with different types of assertions on classes, is presented. The assertions are used to model IS-A and disjointness relations both between entities and between relationships, part-of relations between entities and relationships, mandatory participation of an entity in a relationship, and interdependencies between the projections of relationships. The semantics of the model are defined in terms of first-order logic, and a sound and complete inference algorithm for such a model is presented.