Page 141 - Informatics_Practices_Fliipbook_Class12
P. 141
Relation Works_On
Proj_No Emp_Id Hours
1 E0001 23
2 E0005 18
3 E0002 4
2 E0002 4
4 E0010 30
5 E0009 10
4 E0007 8
1 E0003 17
1 E0008 21
4 E0006 25
Attribute: A column in a table.
Value in a column: Must confirm to Attribute type.
A relational database management system (RDBMS) stores data in the form of relations, also called tables. A table has
can be characterised as follows:
1. Each column of a table represents a unique attribute. For example, the table EMPLOYEE comprises ten columns
representing ID, FName, LName, Gender, Address, City, Pin_Code, DOB, Salary and Dept_No.
Similarly, the table DEPARTMENT comprises attributes, Dept_No, Dept_Name, Location, and Mgr_Id, the
table PROJECT comprises the attributes, Proj_No, Proj_Name, and Dept_Num, and the table WORKS_ON
comprises attributes, namely, Proj_No, Emp_Id, and Hours. One row of a table stores information about all
attributes of an entity. For example, the following row of table EMPLOYEE stores information about an employee
having ID E0005, FName Anshul, LName Verma, Gender M, Address House 10, Sector 16,
Gautum Budh Nagar, City Noida, Pin_Code 201304, DOB 01-Jan-1990, Salary 100000 and
Dept_No 1.
E0005 Anshul Verma M House 10, Sector 16, Noida 201304 1990-01-01 100000 1
Gautam Budh Nagar
Relational schema: Attribute names, types, constraints
2. Relational schema describes the structure of a relation. It includes the names of attributes of a table, types of
attributes, and the constraints on a table. Consider the relation schema of EMPLOYEE relation:
EMPLOYEE(ID, FName, LName, Gender, Address, City, Pin_Code, DOB, Salary, Dept_No).
The names of the relations and the attributes must relate to the attribute's real-world meaning. For example,
FName and LName denote the first name and last name in the EMPLOYEE relation, respectively. The rest of the
attribute names are self-explanatory.
Degree of a relation: No of attributes
The number of attributes in a relation is known as the degree of relation. For example, relation EMPLOYEE has
degree 10, relation DEPARTMENT has degree 4, and each of the relations PROJECT and WORKS_ON has
degree 3.
Database Query using SQL 127

