Elements Of ER Model
Elements Of ER Model: We know that E-R model describes the data as entities, attributes and relationship between entities. So the major elements of E-R model are entities, associated attributes of entities and relationship.
The elements of E-R model are
The basic object that the E-R Model represents is an entity, which is “a thing’’ in real world with an independent existence which is the most important item in the data file of E-R Model. You can also say that anything in a real world that has a set of different attributes or properties is known as entity. An entity can be an object with a physical existence such as person, place, car, computer, house, student and a teacher.
Each entity type is given a unique name. In E-R diagram a rectangular box is used to represent the entity. The name of the entity is written inside the rectangle in the E-R Model.
Entity instance is a single occurrence of an entity type. In other words, you can say that a member of an entity class is called entity instance. An entity type is described just once in a database, while many instance of an entity type has its own values.
Each entity has a set of attributes associated with it. An attribute is a named property or characteristic of an entity that describes it in the E-R Model of the data file of the organization. An entity may have many attributes. For example, a student may have the following attribute name, roll number, height, class, address, color etc. In E-R diagram an attribute is represented by placing its name in an ellipse shape or oval shape.
Types of Attributes
There are several types of attributes that can occur in E-R model. Some of them are
- Simple Attribute
- Composite Attribute
- Single-Valued attributes
- Multi-Valued Attributes
- Derived & Stored Attribute
The attribute of an entity that cannot be divided into smaller components are called simple attribute. They are also referred to as atomic attributes. For example the age, color and roll number of a student. It is one of the famous element in the Elements of ER Model.
The attribute of an entity that can be divided into smaller component is called composite attribute. Each component also represents simple attribute with independent meaning in the elements of the E-R Model of the same data file. For example student’s name can be divided into first name, middle name and last name. Similarly student’s address can be divided into Zip code, city, street address etc.
The attribute that has a single value for each entity instance is called single-valued attribute of the elements of E-R Model of the same organization. For example, the age of the student is a single value attribute
The attribute that can have more than one value for each entity instance is called multi-value attribute. For example a student may more than one phone number, addresses or master degrees.
It can be shown as double ellipse shaped.
Derived & Stored Attribute
The attribute that are stored in a database are called stored attribute or base attribute, while the attribute that are derived from the stored attribute are called derived attribute.