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Relational Data Model

The Relational Data Model, after more than a decade, has emerged as a commercial product from the research, development, test and trial stages. It represents the database as a collection of relations, which resembles a table of values. Each row of the table represents a collection of related data values. The table name and column names are used to help in interpreting the meaning of values in each row. Each row of a table is called a data record.

In this model, the relation is the only link required to represent the associations among the attributes of an entity as well as the relationships among different entities.

One of the main reasons for introducing this model was to increase the productivity of the application programmer by eliminating the need to change application programs when a change is made to the database.

Let us take the following table of data items with respect to accounts as an example. The table consists of rows and columns. A column called Name has code and type in the row. The row arrow points to a data record consisting of (1100001, Capital Account and 1), each of which corresponds to Code, Name and Type, which form three different columns of the table. An example of a data table and its attribute values is as follows-

Name of Table - Accounts






Capital Account



Capital Account



M/s Raj Distributors



Office Maintenance Account



Staff Welfare Account



Rows of the relation are referred to as tuples of the relation, the columns are its attributes and the table its relation. For example, accounts is the name of the relation having three attributes - identity of account (through code), name of account and type of account i.e., asset, liability, expense or income. Each attribute of a relation has a distinct name. The values for an attribute or a column are drawn from a set of values that the attribute may assume. A relation is a set of tuples wherein each tuple is an ordered list of values corresponding to the attributes of relation. Each tuple in the relation represents a particular entity. A relation schema may be interpreted as a declaration in the nature of an assertion. Each value in a tuple is an indivisible value which implies that it is not divisible into components within the framework of the basic relational model.

Relational Databases and Schemas

A Relational Database Schema is made up of a number of relational database schemas and also a set of integrity constraints. A relational database is based ion relation scheme consisting of a number of attributes. It is a set of relation states such that every relational database state satisfies the integrity constraints specified on relational database schema. From another point of view, a relation is a subset of the Cartesian product of the domains.

Each tuple in a relation represents an identifiable instance of an object type. But it should also be considered that the set of tuples in the relation are not static but can vary with time. The set of attributes on which the tuples are defined is the invariant. This is called the scheme of the relation or the relation scheme. The relation has two parts - a relation scheme and a time - varying set of tuples.

A Relational Database Schema comprises of -
  1. The definition of all domains
  2. The definition of all relations, specifying for each
    1. its intension and
    2. a primary key
However, there is a useful notation for relational schemas commonly adopted to document and communicate database designs free of any specific DDL. It takes the simple form:

<relation name>:<list of attribute names>

Additionally, attributes that are a part of the primary key are underlined. Thus, for the example in Exhibit 1.21 the schema would be written as follows:


Customer : (C#, C Name, Ccity, Cphone)
Transaction : (C#, P#, Date, Qnt)
Product : (P#, Pname, Price)


This notation is useful in clarifying the overall organization of the database but omits some details, particularly the properties of domains. As an example of a more complete definition using a more concrete DDL, we rewrite some of the schema about using Amar's original notation. The principal components of his notation are annotated alongside.

Database State and Instances: The data in the database at a particular moment is called database state or snapshot. This is also called current set of occurrence or instances.

When a new database is defined, its state is empty. The initial state of database occurs only when the database is first populated or loaded with initial data.

Data Models based on Data Structure

Relational Data Model

Hierarchical Data Model

Network Data Model

Data is organized into rows and columns. These tables are called relations. A row in a table represents a relationship among a set of values.

Data is represented by a collection of records and relationship among data is represented by links as the records are organized in a tree structure rather than as an arbitrary graph

Data is represented by collection of records and the relationship among data is represented by links. A record is a collection of field(attributes) each of which contains only one data value.

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