Module 0 - Introduction

What is a database?

A database is a collection of related data or known facts that:

• Represents some aspect of the real world, sometimes called the mini-world or the universe of discourse (UoD).
• Is a logically coherent collection of data with some inherent meaning.
• Is designed, built, and populated with data for a specific purpose for an intended group of users and some preconceived applications.

Components

From bottom to top:

1. The Database, the physical storage of data
2. The DBMS - the systems responsible for the definition, construction and manipulation of the Database
3. Applications and user views of data, either through the DBMS interface or through application programs

Features of a DBMS:

• Data Integrity Maintenance
  • DBMS has the capability to define and enforce integrity constraints
• Query Processing
  • A DBMS is equipped with a programming language to run queries and return data.
• Security Management
  • DBMS's provide various measures for securing databases against a variety of threats.
• Concurrency Control
  • DBMS utilises control protocols to handle concurrent access to data files that guarantee serialisability (think like google docs)
• Backup and Recovery
  • DBMS provides facility to recover from hardware and software failures through its backup and recovery sub-system

Three schema architecture - see also Javapoint.com

• External Level: provides access to particular parts of the database to users
• Conceptual Level: describes the structure of the whole database for a community of users.
• Internal Level: describes the physical storage structure of the database

Logical Data Independence: Ability to change the conceptual schema without changing external views or applications

To Add/Modify/Delete a new attribute, entity or relationship is possible without a rewrite of existing application programs

Physical Data Independence: Ability to modify physical schema w/o changing conceptual schema

E.g. Move our physical database somewhere else (move onto cloud or another data warehouse), without changing our conceptual level schema

Still confused?

• It may be instructive to view the Three-Schema Architecture as a model rather than as a hard and fast rule. In such as way, data independence is an objective to be achieved, rather than something that does work.