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Database system is a computer based record keeping system.
It is a system whose overall purpose is to record and maintain information. The information concerned can be anything that is deemed important to the organization.
In another words, database is collection of stored operational data which can be used and shared by different applications and users of any enterprise. This enterprise could be scientific, commercail, educational, etc.
Database system provides the enterprise with centralized control of its operational data, which is one of its most valuable assets.
This is totally oposite of the situation that is happening in many organizations today, where typically each application has its own private files and a lot of times its own private tapes and disk packs too. This makes the operational data widely dispered and difficult to control.
Redundancy can be reduced Inconsistency can be avoided Data can be shared Standards can be enfored Security restrictions can be applied Integrity can be maintained Conflicting requirements can be balanced
Data independence Data indepencence is an objective in database development Most present day applications are data dependent. This is
the way in which the data is organized in secondary storage and is accessed. Both are dictated by the requirements of the application. This is undesirable because:
» Different applications will need defferent view of the same data.
» Data Base Adminstrator (DBA) must have the freedom to change the storage structure or access strategy or both in response to changing requirements without the necessity of modifying existing application.
Each user communicates with the database via a language referred as data sub-language (DSL), which is the subset of the language concerned with the retrieval and storage of information in the database.
Mapping defines the correspondence between data models and external/internal schema.
DBMS is the software that handles all access to the database and also is responsible for applying the authorization checks and validation procedures.
Data dictionary is the most important tool for DBA. Data dictionary is a database on its own right. It defines all data and their relationship, “ data anot