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Unformatted text preview: of a class-composition
hierarchy involving the class Employee.
For the database structure of Figure 16.11, a typical query may be like "list out the
President's and Company's names for all the companies located in Pune, India that
manufacture two-wheeler vehicles."
Main Components of a DBMS
A DBMS frees the users of a database from the need to worry about the
organization and location of data. It allows users to organize, process, and retrieve
selected data from a database without knowing about the underlying database
structure. The four major components of a DBMS that enable this ease of use of a
database are: 1. Data Definition Language (DDL),
2. Data Manipulation Language (DML)
3. Query Language, and
4. Report Generator.
They are briefly described below. - Data Definition Language (DDL)
The data definition language (DDL) is used to define the structure of a database.
The database structure definition (also commonly known as database schema)
typically includes the following:
1. Defining all data elements to be included in the database.
2. Defining the organization of the data elements (fields) into records (or tuples),
3. Defining a name, field length, and field type for each data element. A field
name is used to refer to the corresponding data element while accessing it. Field
length is used to define the maximum size c the data element (such as an employee
name field, may have a maximum size of 30 characters Common field types are
numeric (can contain only the digits 0 to 9, a decimal point, and a + or sign),
alphanumeric (can contain a combination of alphabetic characters, special
symbols, and digits logical (can contain one of two possible values - Yes/No or
True/False), date (can contain a single date), and memo (can be used to hold any
type of reminder that the user might like to type).
Defining controls for fields that can have only selective values.
example, in an employee database, the sex field can have controls to accept only
the values M or F. Similarly, the age field can have controls to accept only the
values 18 or more and less than 70. Such controls ensure the correctness of
entered data to some extent.
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This document was uploaded on 04/07/2014.
- Spring '14