Entity-relationship diagram
Entity-relationship diagram is one of the semantic data modeling tools. It can be used to link data within a diagram at the conceptual level and thus illustrate and store it abstractly. It serves as the basis for modeling a database system and is also referred to as a blueprint for the database to be designed. The Entity-relationship diagram uses relationships to link different entities with each other and describes them with the corresponding attributes. The areas in which the Entity-relationship diagram is used and how it is structured are explained in more detail in the following article[1].
In 1976, the Entity-relationship model was first introduced by the American computer scientist Peter Chen. In his publication, the scientist detailed the structure of the model and introduced a special diagramming technique, the Entity-relationship diagram. Since then, this has become one of the most common forms of semantic data modeling[2].
The use of a semantic data model
The collection, storage and visualization of data lead to knowledge and new knowledge can always be derived from existing knowledge. Database systems represent a technical possibility for managing knowledge in the form of data. The conceptual view of data can be represented with the help of semantic data modeling. This means that information from the real world is defined with the help of symbols. The model thus represents an abstraction of the real world and must correspond to a faithful representation of it. A semantic data model can be used for different purposes, which are listed below[3]:
- use to be able to plan data resources: create an overall view to achieve an understanding of data and conduct analysis of the model to build future data resources
- use to be able to build databases: define the conceptual view of data, then have it validated by customers for transformation into a database management system
- use to be able to organize and document data: the creation of an Entity-relationship diagram to be able to subsequently implement the data model in a database
Structure of an Entity-relationship diagram
An Entity-relationship diagram consists of entities, their descriptive attributes and the linking relationships. To illustrate the structure, a sketch of an Entity-relationship diagram is shown on the right side. The entities are each shown in a rectangle and the entity name is inside this rectangle. The relationship is symbolized by a diamond and connects the two entities. Often the descriptive verb of the relationship is written inside the diamond. The cardinality of the relationship is between the diamond and the rectangle. Descriptive attributes of an entity are noted inside an ellipse and connected to the respective entity[4].
Entities: Within an Entity-relationship diagram, entities represent important data objects. These are real or abstract objects such as people, places, things, or events that are of high interest to the business and about which data should be stored[5].
Relationships: Pre-defined entities are connected with the help of logical links. These links can also be described as a relationship or association between the entities. With the help of different verbs, such as buy, deliver or produce, the relationship between the entities can be defined more precisely[6].
Furthermore, within the Entity-relationship model, there are three different connectivities of how entities can be linked to each other[7]:
- one to one relationship (1:1):
One element of entity A can be associated with exactly one element of entity B.
The figure shows the employee associated with the business car. One employee can drive one company car and one company car is driven by one employee.
- one to many relationship (1:N):
One element of entity A can be assigned to multiple elements of entity B. One element of entity B can be associated with exactly one element of entity A.
The figure shows the employee associated with the department. One department can include several employees but one employee can be associated with only one department.
- many to many relationship (N:M):
One element of entity A can be associated with multiple elements of entity B and vice versa.
The figure shows a project that is associated with employees. Several employees can work on one project. And one employee can work on several projects.
Attributes: Attributes are typical properties that describe an entity. The attribute by which an entity can be identified is called a key attribute. For example, if the entity customer is given, the customer_ID represents the key attribute and the other attributes such as customer name, customer address, etc. represent descriptive characteristics of the entity[8].
Why should companies use the Entity-relationship diagram?
The Entity-relationship diagram is a simple graphical method to illustrate information of interest to the business. Its simplicity makes the diagram understandable not only to experienced experts such as systems analysts but also to inexperienced employees and managers. Thus, the Entity-relationship diagram is an ideal communication tool between system analysts and managers during the process of determining user information requirements[9].
Footnotes
| Entity-relationship diagram — recommended articles |
| Knowledge structure — Business process modeling — Matrix diagram — Business process mapping — IT management system — Types of knowledge — Logic matrix — Database — Internet |
References
- Bagui, S., Earp, R. (2011). Database Design Using Entity-Relationship Diagrams. Auerbach Publishers.
- Chen, P.P.S. (1976). The entity-relationship model—toward a unified view of data. ACM Transactions on Database Systems. 1(1), 9-36.
- Chen, P.P.S. (1983). English sentence structure and entity-relationship diagrams. Information Sciences, 29(2-3), 127-149.
- Li, Q., Chen, YL. (2009). Entity-Relationship Diagram. In: Modeling and Analysis of Enterprise and Information Systems. Springer, Berlin, Heidelberg.
- Teorey, T. J., Lightstone, S. S., Nadeau, T., Jagadish, H., Teorey, T. J., Lightstone, S. S., Nadeau, T., & Jagadish, H. (2011). Database modeling and design : Logical design. Elsevier Science & Technology.
Author: Andrea Adkins
