Database and DBMS

Database:
 It is a collection of related data that contains information about one particular enterprise.

DBMS (Database Management System): It is a software system that allows data contained in a database. The objective of DBMS is to provide an easy method of defining, storing and retrieving information contained in database.
Characteristics of database approach:
In the database approach a single repository of data is maintained that is define once and then accessed by various users. In file systems, each application is free to name that are elements independently. In contrast in a database the names or levels of data define once and used used repeatedly by queries, transactions, applications. The main characteristics of database approach versus the file processing approach are the following:

• Self describing nature of a database system
• Insulation between programs and data, data abstraction
• Support of multiple views of the data
• Sharing of data and multi user transaction processing.

Advantages of DBMS:
1) Reduction of data redundancy ( data redundancy means duplication of data)
2) Sharing of data
3) Integrity ( integrity means data quality enhanced)
4) Security
5) Data independence : we see this from two point of view;
a) physical data independence
b) logical data independence

Disadvantages of DBMS: 
DBMS has main three disadvantages:

1) Problem associated with centralisation
2) Cost of software/hardware and migration
3) Complexity of back up and recovery

Structure of DBMS: 
 The major components of the DBMS are:
1) Data definition language compiler (DDL)
2) Data Manager
3) File Manager
4) Disk Manager
5) Query processor
6) Database Abstraction: The data is abstracted in three levels:

• External View 
• Conceptual or Global View
• InternalView

The three-schema architecture of DBMS:

1.) Internal level: The internal level has an internal schema which describe the physical storage structure of the database. The internal schema uses a physical data model and describes the complete details of data storage and access paths for the database.

2.) Conceptual Level: The conceptual level has a conceptual schema, which describes the structure of the whole database for a community of users. The conceptual schema hide the detail of physical storage structures and concentrate on describing entities, data types, relationships, user operations and constraints.

3.) External Level: The external view or level includes a number of external schemas or user views. Each external schema describes the part of the database that a particular user group is interested in and hides the rest of the database from that user group.