Introduction -: Now a days relation data model is being used as the primary model for commercial data processing application. It was introduced by E.F.Codd in 1970 and it changed permanently the way people viewed database.
The relation model is based on logical relationship between data and allows user to be totally unconcerned with or even unaware of the physical structure of the data.
Concept of Relational Data Model-: The Relation data Model organise and represent data in the form of tables or relation. Relation is a term that comes for mathematics and represents a simple two dimension table,consisting of rows and column of data.
A row in a table represent a relationship among set of values which may be related to an object or entity. The row of a relation are also called a tuples.It is assumed that there is no prespecified order of the rows or tuples or a relation and that no two tuples have identical set of values. Each column in the relation is an attribute of the relation. The name of the column is attribute name . The set of all possible values that an attribute can have is the domain of the attribute and total number of attribute of a relation is called degree .
As like in a company database where we store data inn 3 table Employee, Department, and dependent having following schema
In Employee table we define the
- Empno : Number
- Ename : String
- Designation : String
- Mgr No : Number
- Date of Joining : Date
- Salary : Number
- Dept no. : Number
In the Department table we define the data in this way
- Dept no : Number
- Dname : string
- Hod : number
Dependent
- Empno : Number
- DepetNo :Number
- Deptname : string
- Age :Number
We now turn to the instance of these relation
Employee relation ha a unique field ’empno’ which is used to distinguished between different employee. This field is Unique as no two tuple of employee relation can have save empno also note that deptno field of employee relation is used to link employee information with his department Information and all the value in deptno field are from the deptno field of department table as an employee can work only for a department that exists in department tables.