Database management system by Neeraj Bhandari ( Surkhet.Nepal )

Database
A database is an organized collection of data. The
data are typically organized to model relevant
aspects of reality (for example, the availability of
rooms in hotels), in a way that supports
processes requiring this information (for
example, finding a hotel with vacancies).
Is a structured collection of records or data that is
stored in a computer system.
The term database system implies that the data
are managed to some level of quality (measured
in terms of accuracy, availability, usability, and
resilience) and this in turn often implies the use
of a general-purpose database management
system (DBMS).

Database management
System
A general-purpose DBMS is typically a
complex software system that meets
many usage requirements to properly
maintain its databases which are often
large and complex.

Uses
 Increase productivity through real-time component
data and design re-use.
 Consolidate parts, inventory and manufacturing
requirements.
 Decision support through integration with
enterprise business systems applications.
 Information systems can be changed easily
according to the company's requirements.

Examples
 Oracle
 Microsoft Access
 Microsoft SQL server
 Firebird
 FileMaker

Applications
 computerized library systems
 automated teller machines
 flight reservation systems
 computerized parts inventory systems

Basic Definitions
Attribute - a property or description of an entity. A toy
department employee entity could have attributes
describing the employee’s name, salary, and years of
service.
Domain - a set of possible values for an attribute.
Entity - an object in the real world that is distinguishable
from other objects such as the green dragon toy.
Entity set - a collection of similar entities such as all of the
toys in the toy department.
Key - A key is an attribute (also known as column or field)
or a combination of attribute that is used to identify
records. Sometimes we might have to retrieve data
from more than one table, in those cases we require to
join tables with the help of keys. The purpose of the key
is to bind data together across tables without repeating
all of the data in every table.

Data Viewing
External, logical and internal view
 A DBMS Provides the ability for many different users to share data
and process resources. As there can be many different users, there
are many different database needs. The question is: How can a
single, unified database meet varying requirements of so many
users?
 A DBMS minimizes these problems by providing three views of the
database data: an external view (or user view), logical view (or
conceptual view) and physical (or internal) view. The user’s view of a
database program represents data in a format that is meaningful to a
user and to the software programs that process those data.
 One strength of a DBMS is that while there is typically only one
conceptual (or logical) and physical (or internal) view of the data,
there can be an endless number of different external views. This
feature allows users to see database information in a more
business-related way rather than from a technical, processing
viewpoint. Thus the logical view refers to the way the user views the
data, and the physical view refers to the way the data are physically
stored and processed.

DDL (Data Definition Language)
It is used to create and destroy databases and database objects. These
commands will primarily be used by database administrators during the
setup and removal phases of a database project.
Commands:
CREATE: Installing a database management system (DBMS) on a
computer allows you to create and manage many independent
databases. For example, you may want to maintain a database of
customer contacts for your sales department and a personnel database
for your HR department. The CREATE command can be used to
establish each of these databases on your platform. For example, the
command:
CREATE DATABASE employees
creates an empty database named "employees" on your DBMS.
USE: The USE command allows you to specify the database you wish to work with
within your DBMS. For example, if we're currently working in the sales database
and want to issue some commands that will affect the employees database, we
would preface them with the following SQL command:
USE employees

ALTER: Once you've created a table within a database, you may wish to modify the
definition of it. The ALTER command allows you to make changes to the structure of a
table without deleting and recreating it. Take a look at the following command:
ALTER TABLE personal_info
ADD salary money null
This example adds a new attribute to the personal_info table -- an employee's salary.
The "money" argument specifies that an employee's salary will be stored using a dollars
and cents format. Finally, the "null" keyword tells the database that it's OK for this field to
contain no value for any given employee.
DROP: The final command of the Data Definition Language, DROP, allows us to remove
entire database objects from our DBMS. For example, if we want to permanently remove
the personal info table that we created, we'd use the following command:
DROP TABLE personal_info
Similarly, the command below would be used to remove the entire employees database:
DROP DATABASE employees
Use this command with care! Remember that the DROP command removes entire data
structures from your database. If you want to remove individual records, use the
DELETE command of the Data Manipulation Language.

DML(Data Manipulation
Language)
It is used to retrieve, insert and modify database information. These
commands will be used by all database users during the routine
operation of the database.
Commands:
INSERT
The INSERT command in SQL is used to add records to an existing
table. Returning to the personal_info example from the previous
section, let's imagine that our HR department needs to add a new
employee to their database. They could use a command similar to
the one shown below:
INSERT INTO personal_info
values('bart','simpson',12345,$45000)
Note that there are four values specified for the record. These
correspond to the table attributes in the order they were defined:
first_name, last_name, employee_id, and salary.

SELECT
The SELECT command is the most commonly used command in SQL. It allows
database users to retrieve the specific information they desire from an operational
database. Let's take a look at a few examples, again using the personal_info table
from our employees database.
The command shown below retrieves all of the information contained within the
personal_info table. Note that the asterisk is used as a wildcard in SQL. This
literally means "Select everything from the personal_info table."
SELECT *
FROM personal_info
Alternatively, users may want to limit the attributes that are retrieved from the
database. For example, the Human Resources department may require a list of
the last names of all employees in the company. The following SQL command
would retrieve only that information:
SELECT last_name
FROM personal_info
Finally, the WHERE clause can be used to limit the records that are retrieved to
those that meet specified criteria. The CEO might be interested in reviewing the
personnel records of all highly paid employees. The following command retrieves
all of the data contained within personal_info for records that have a salary value
greater than $50,000:
SELECT *
FROM personal_info
WHERE salary > $50000

UPDATE
The UPDATE command can be used to modify information contained within a
table, either in bulk or individually. Each year, our company gives all employees a
3% cost-of-living increase in their salary. The following SQL command could be
used to quickly apply this to all of the employees stored in the database:
UPDATE personal_info
SET salary = salary * 1.03
On the other hand, our new employee Bart Simpson has demonstrated
performance above and beyond the call of duty. Management wishes to recognize
his stellar accomplishments with a $5,000 raise. The WHERE clause could be
used to single out Bart for this raise:
UPDATE personal_info
SET salary = salary + $5000
WHERE employee_id = 12345
DELETE
Finally, let's take a look at the DELETE command. You'll find that the syntax of this
command is similar to that of the other DML commands. Unfortunately, our latest
corporate earnings report didn't quite meet expectations and poor Bart has been
laid off. The DELETE command with a WHERE clause can be used to remove his
record from the personal_info table:
DELETE FROM personal_info
WHERE employee_id = 12345

Database management system by Neeraj Bhandari ( Surkhet.Nepal )

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Database management system by Neeraj Bhandari ( Surkhet.Nepal )