Introduction to Database System

• A database is a collection of related data.
• By data,we mean known facts that can be recorded and that have implicit meaning.
• A database can be of any size and complexity. For example, the list of names and
  addresses referred to earlier may consist of only a few hundred records, each with a
  simple structure.

A database has the following implicit properties:

• A database represents some aspect of the real world, sometimes called the miniworld or the universe of discourse (UoD). Changes to the miniworld are reflected in the database.
• A database is a logically coherent collection of data with some inherent meaning. A random assortment of data cannot correctly be referred to as a database.
• A database is designed, built, and populated with data for a specific purpose. It has an intended group of users and some preconceived applications in which these users are interested.
  

An example of a large commercial database is Amazon.com

It contains data for over 20 million books, CDs, videos, DVDs, games, electronics, apparel, and other items. The database occupies over 2 terabytes (a terabyte is 1012 bytes worth of storage)
and is stored on 200 different computers (called servers). About 15 million visitors
access Amazon.com each day and use the database to make purchases. The
database is continually updated as new books and other items are added to the
inventory and stock quantities are updated as purchases are transacted. About 100
people are responsible for keeping the Amazon database up-to-date.

A database management system (DBMS)

A database management system (DBMS) is a collection of programs that enables
users to create and maintain a database. The DBMS is a general-purpose software system
that facilitates the processes of defining, constructing, manipulating, and sharing
databases among various users and applications.

• Defining a database involves specifying
  the data types, structures, and constraints of the data to be stored in the database.
• Constructing the database is the process of storing the data on some storage medium that is controlled
  by the DBMS.
• Manipulating a database includes functions such as querying
  the database to retrieve specific data, updating the database to reflect changes in the
  miniworld, and generating reports from the data.
• Sharing a database allows multiple users and programs to access the database simultaneously.

Other important functions provided by the DBMS include protecting the database
and maintaining it over a long period of time. Protection includes system protection
against hardware or software malfunction (or crashes) and security protection
against unauthorized or malicious access. A typical large database may have a life
cycle of many years, so the DBMS must be able to maintain the database system by
allowing the system to evolve as requirements change over time.
we will call the database and DBMS software together a database system.