2.1 What is a Database Management System (DBMS)

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1.1 DBMS definition

DBMS is a collection of software for creating, managing and effectively searching a database.

Figure 2.1 illustrates the role of a DBMS hosting a database and controlling storage and access to its data. In addition a DBMS is responsible for managing application requests to a database.

2.1 What is a Database Management System (DBMS)

1.2 DBMS Three-Schema Architecture

DBMS architecture could be described based on three abstraction levels architecture as shown in Figure 2.2. Such architecture ensures the independence of logical and physical data, allowing the manipulation of data, ensure data integrity and optimize data access. The three levels of the three-schema architecture of are described below:

a) External layer or External Schema it covers all the possibilities of access to data by different users. Such access of different user groups can be done via different applications. Each external schema describes the part of the database that a particular user is interested in and hides the rest of the database from user.

b) Logical Layer or Logical Schema It is the general view of the data independent of individual applications and how data is stored.

It hides the details of physical storage structures and concentrates on describing entities, data types, relationships, user operations, and constraints.

c) Internal layer or Internal Schema This layer describes the physical storage structure of the database. The internal schema describes the complete details of data storage and access paths for the database.