High-Level or Conceptual Data Model
The conceptual data model is a description of the data requirements of the user. This model is not concerned with the implementation details. It ensures that all the functional and data requirements of the users are specified, conceptually. The conceptual model is defined using terms like (1) Entity, (2) Attribute, and (3) Relationship. The Entity-Relationship model (E-R model) is an example of conceptual data model. The following subsections briefly describe these terms and the E-R model.
Entity
An entity is the basic unit for modelling. It is a real-world object that exists physically or conceptually. An entity that exists physically is a tangible object like student, employee, room, machine, part or supplier. An object that exists conceptually is a non-tangible object like an event or job title. For e.g. student information system may consist of entities like student profile, marks and course. A set of entities of the same type having same properties, or attributes is defined as an entity set. For example, a set of all persons who are students of the university can be defined as an entity set student. Likewise, the entity set course may represent a set of all courses offered by a University. An entity set is usually referred to by the same name as an entity. For example, Student is an entity set of all student entities in the database.
Diagrammatically, an entity is represented using a rectangle. Figure shows the entities for a database created in MS-Access 2007.
Attribute
An attribute describes some property or characteristics of the entity. For e.g. student name, student address, and student age are attributes of the entity student profile. An attribute ensures that similar information is stored about each entity in an entity set, in the database. However, each attribute of an entity may have its own value. The set of permitted values for an attribute is called the domain of the attribute. For example, the domain of the attribute student name is text string, or, the domain of student age is a positive integer ranging between 18 and 65. Diagrammatically, an attribute is represented as an ellipse connected to the entity with a line. Figure shows the attributes for the entity “Employee” in MS-Access 2007.
Relationship
An association or link between two entities is represented using a relationship. A set of relationships of the same type form a relationship set. For e.g. “student enrolls in course” is a relationship set between the entities student and course. Cardinality ratio is the number of entities to which another entity gets associated in a relationship set. The cardinality ratio of a relationship set is any one of the four kinds—(1) One-to-One, (2) One-to-Many, (3) Many-to- One, and (4) Many-to-Many. In two entity sets A and B, the different cardinality ratio imply the following:
· One-to-One: An entity in A is associated with at most one entity in B and vice versa.
· One-to-Many: An entity in A is associated with any number of entities in B, but an entity in B is associated with at most one entity in A.
· Many-to-One: An entity in A is associated with at most one entity in B, but an entity in B is associated with any number of entities in A.
· Many-to-Many: An entity in A is associated with any number of entities in B and vice versa.
Diagrammatically, a relationship is represented using a diamond connected to the entity by a line. Figure shows the different kinds of relationships.
Entity-Relationship (E-R) Model
E-R model is a model of the real world. E-R model represents the entities contained in the database. The entities are further described in the database using attributes. The relation between the entities is shown using the relationships. The model also shows the cardinality constraints to which the database must adhere to. The E-R model is represented diagrammatically using an E-R diagram. Figure shows a simple E-R diagram. The diagram shows two entities—Student and Course. The Stud_Name and Stud_RollNo are the attributes of the entity Student. The Course_Id and Course_Name are the attributes of the entity Course. The Admission relationship associates the student with the course. Database design in E-R model is converted to design in the Representation Model which is used for storage and processing.
Relationships (a) One-to-one (b) One-to-many (c) Many-to-many
E-R diagram
Object-Oriented (OO) database model use object-oriented programming. The object data model defines a database in terms of objects, classes, and methods. It is beneficial in web applications, and in specialized application areas like multimedia, manufacturing system, and engineering design, and for the storing of multimedia objects.