The Relational Database Management System is based on relational model proposed by E.F Codd in 1969. Relational Model has been the most preferred foundation used in developing commercial data processing applications. The reason for its wide acceptance is the simplicity of the relational model. It gives the capability to represent the real world data in the form of relations and their relationships. It is a better model when compared with network model or hierarchical model. The database designer can understand and design the database structure in a way closer to the real world.
Basic Terms used in Relational Database Management System
In a relational model certain terms are used to describe the objects or features of the database.
A relation is the basic unit of data in a Relational Database Management System that represents an entity of the real world. It can be a person, an object or an event. In a customer database the relations can be customer, items, invoices, orders etc.
Attributes are the features of a relation. In the customer database the customer relation is described by these features- Customer ID, Customer Name, Customer address, Customer Contact number, Customer Type etc. Attribute names cannot be duplicated in a relation.
Tuples represent the data stored in the relation. One tuple can be understood as one instance of all the attributes of a relation when assigned the permitted values. For example a tuple in customer relation is Customer ID=C101, Customer Name= James Martin, Customer address= 99 Upper Street, Newyork, Customer Contact number= 631-268-4955, Customer Type= Regular.
Domain is the set of all values for an attribute from which one value can be assigned for one tuple or instance. For example in customer relation the customer ID can be any value from C101 to C999.
A relation instance is the specific instance of a relation at any moment of time represented by a specific set of tuples.
Degree (of a relation)
Degree is always related to a specific table in a relational database defined as the total number of attributes that defines the structure of a relation. In the example discussed here the degree of customer relation is five.
Cardinality ( of a relation)
This again is a property of a relation specifying the total number of tuples the relation contains at any instance.
Key (of a relation)
A key of the relation is an attribute or a group of attributes that help in uniquely identifying tuples in a relation.