Entity-Relationship Model (E-R Model)

Underlying the structure of a database is the data model, a collection of conceptual tools for describing data, data relationships, data semantics, and consistency constraints. We are going to discuss one of the Data Model in this Article. 

The Entity-Relationship Model:

The entity-relationship (E-R) data model is based on a perception of a real-world that consists of a collection of basic objects, called entities, and of relationships among these objects. An entity is a “thing” or “object” in the real world that is distinguishable from other objects. For example, each person is an entity, and bank accounts can be considered as entities.

Entities are described in a database by a set of attributes. For example, the attributes account-number and balance may describe one particular account in a bank, and they form attributes of the account entity set. Similarly, attributes customer-name, customer-street address, and customer-city may describe a customer entity.

An extra attribute customer-id is used to uniquely identify customers (since it may be possible to have two customers with the same name, street address, and city). A unique customer identifier must be assigned to each customer. In the United States, many enterprises use the social security number of a person (a unique number the U.S. government assigns to every person in the United States) as a customer identifier.

A relationship is an association among several entities. For example, a depositor relationship associates a customer with each account that she has. The set of all entities of the same type and the set of all relationships of the same type are termed an entity set and relationship set, respectively.

The overall logical structure (schema) of a database can be expressed graphically by an E-R diagram, which is built up from the following components:

Rectangles, which represent entity sets

Ellipses, which represent attributes

Diamonds, which represent relationships among entity sets

Lines, which link attributes to entity sets and entity sets to relationships

Each component is labeled with the entity or relationship that it represents.

As an illustration, consider part of a database banking system consisting of customers and of the accounts that these customers have. The below diagram shows the corresponding E-R diagram. The E-R diagram indicates that there are two entity sets, customer and account, with attributes as outlined earlier. The diagram also shows a relationship depositor between customer and account.

In addition to entities and relationships, the E-R model represents certain constraints to which the contents of a database must conform. One important constraint is mapping cardinalities, which express the number of entities to which another entity can be associated via a relationship set. For example, if each account must belong to only one customer, the E-R model can express that constraint.

The entity-relationship model is widely used in database design.

In our next article, we will discuss the other type of Data model "Relational Data Model".