Introduction To Databases

Definition

A computer database is a structured collection of records or data that is stored in a computer system. A database relies upon software to organize the storage of the data and to enable a person or program to extract desired information. The term database refers to the collection of related records, and the software should be referred to as the database management system DBMS.

Purpose of Databases

Example: A typical file-processing system for a savings bank.
Savings accounts and customer records are kept in permanent system files. Application programs are written to manipulate files to perform the basic tasks: FIND, ADD, DELETE.
But over the long period files may be in different file formats and application programs may be in different languages.
The problems are:

  • Data redundancy and inconsistency
  • Difficulty in accessing data
  • Data isolation: Data in different files and in different formats.
  • Multiple users: Concurrent updates may cause problems.
  • Security problems: Difficult to enforce with application programs.

Data Abstraction

The major purpose of a database system is to provide users with an abstract view of the system. The system hides certain details of how data is stored and maintained.

Levels of abstraction:

  1. Physical Level: How the data are stored.
  2. Conceptual Level: Describes what data are stored and the relationship between them.
  3. View Level: Describes part of the database for a particular group of users.

Data Models

Collection of conceptual tools for describing data, data relationships, data semantics and
data constraints. There are three different groups:
Object-based Logical Models
Record-based Logical Models
Physical Data Models

Instances and Schemes

Databases change over time. The information in a database at a particular point in time is called an instance of the database. The overall design of the database is called the database scheme.

Data Independence

The ability to modify a scheme definition in one level without affecting a scheme definition in a higher level
is called data independence.

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