Page 180 - Informatics_Practices_Fliipbook_Class12
P. 180

On executing the above statement, SQL will display the output shown in Table 4.39.


                                                 City           RangeSalary
                                                Delhi              30000

                                             Jamshedpur            40000
                          Table 4.39 City and the range of salary for employees residing in the same city having
                                        the range of salary greater than or equal to 30000.





             Let's Summarise



              Database Management System (DBMS) is a collection of programs that help define, construct, and manipulate
           Ø
              the database.
              Database administrator (DBA)  manages the entire database by acquiring resources (both hardware and
           Ø
              software) and controlling access.

              Disadvantages of DBMS:
           Ø
              •  Initial Cost: Applications based on DBMS involve higher costs of  hardware and DBMS software.
              •  Complexity: As DBMS offers various functionalities, it becomes a large and complex software, requiring
                training of application developers.
              Data Model: A data model is a representation of the structure of real-world objects. A data model comprises
           Ø
              the following components:
              •  Entity: An entity is a real-world object (such as employee and department  in a company) about which data
                is stored in the database.
              •  Attribute: An attribute defines the characteristics of an entity. For example, employee id, date of birth,
                address, and salary form attributes of an employee.
              •  Relationship:  A  relationship  specifies  how  two  entities  are  related  to  each  other.    For  example,  in  a
                ccompany's database  the employee entity is related to the department entity since an employee works in
                a department.
              •  Constraint: A constraint describes the restrictions imposed on the data stored. For example, each employee
                should have a unique AAdhar number.
              Relational  Data  Model:  A  relational  database  management  system  (RDBMS)  stores  data  in  the  from  of
           Ø
              relations, also called tables. A table has the following  characteristics:
              •  Each column of a table represents a unique attribute.

                  (i)  An attribute has a type denoting the type of data in the column.
                  (ii)  Value in a column must conform to the attribute type.
              •  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.
              •  Degree: No of attributes in the column is called degree of the relation.
              •  Tuple: A row in a table is called a tuple.

              •  Entity Set: The set of all tuples at a specific time in a relation is known as the entity set.




          166  Touchpad Informatics Practices-XII
   175   176   177   178   179   180   181   182   183   184   185