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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
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the database.
Database administrator (DBA) manages the entire database by acquiring resources (both hardware and
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software) and controlling access.
Disadvantages of DBMS:
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• 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
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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
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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

