Basic Elements in the E-R Model
Entities, relationships, and attributes are the basic elements or constructs in the E-R model.
Entities
An entity consists of basic objects or concepts about which data is collected and stored. These may be places, people, objects, events, customers, orders, or something abstract like a loan or a holiday. Employees, invoices, projects, etc. are examples of entities. In the relational model, an entity is equivalent to a table.
Entities can be classified based on whether they are independent or dependent. In certain instances, they are also referred to as strong or weak. An independent entity is one which does not depend upon another entity for identification while a dependent entity depends upon another entity for identification. The occurrence of an entity, also known as the instance of an entity, is an individual occurrence of an entity. In the relational model, an occurrence is equivalent to a row while an attribute is equivalent to a column.
There are also special entity types like associative entities and subtype entities. Associative entities or intersection entities are used for associating or relating two or more entities to form a many-to-many relationship. Subtype entities represent a subset of the occurrences or instances of the parent entity, which is called a super type or a generic entity. Subtype entities have attributes or relationships that apply only to the subset.
Relationships
A relationship represents the association between two or more entities. It can be defined as the way in which data is shared between these entities. For example, an organization consists of many departments. Each department consists of a number of employees. Each employee is identified with an employee identification number. Relationships are classified on the basis of degree, connectivity and cardinality, direction, type, and existence.
Degree of relationship: The degree of a relationship refers to the number of entities that are associated with the relationship. Binary relationships, ternary relationships, N-ary relationships, etc. are the different types of relationships classified based on degree. Binary relationships occur between two entities. When an entity is related to itself, the relationship is termed as a recursive binary relationship. Ternary relationships occur between three entities and are used to define situations when binary relationship becomes inadequate. N-ary relationship is the general form of relationship in degree n.
Connectivity and cardinality of relationship: Connectivity refers to the mapping of related entity occurrences in the relationship. These can be one or many. Cardinality refers to the actual number of associated instances for each of the two entities. Three types of relationships based on connectivity take place between entities. These are one-to-one, one-to-many, and many-to-many relationships.
One-to-one or 1:1 relationships take place when one instance of an entity P is associated with one instance of another entity Q. For example, the employee name (P) is associated with the identification number (Q) of the employee. Also, the identification number of the employee can be used to identify the employee name.
One-to-many or 1:N relationships take place when one instance of an entity P is associated with zero, one, or many instances of another entity Q. However, entity Q has an association with only one instance of entity P. For example, a department (P) consists of a number of employees (Q). All these employees (Q) belong to only one department (P). This relationship is also called the many-to-one or N:1 relationship.
Many-to-many or M:N relationships take place when one instance of an entity P is associated with zero, one, or many instances of another entity Q. And, one instance of entity Q is associated with zero, one, or many instances of entity P. For example, in an organization, a single project (P) can be assigned to many employees (Q) and at the same time, a single employee (Q) can be assigned to many projects (P). This relationship is also called the N:M or non-specific relationship.
Attributes
Attributes are characteristics or properties that describe an entity. These characteristics differentiate one entity instance from another. For example, the attributes of an employee include his/her identification number, first name, last name, address, city, state, pin code, phone number, etc.
Attributes can be classified as identifiers or descriptors. Identifiers or keys are special attributes which are used for identifying the unique characteristics of a specific entity. For example, the employee identification number is an identifier that uniquely identifies the employee. Descriptors, on the other hand, identify the non-unique characteristics of an entity.
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