|=--------------------------------------------------=|
. |02.02 - Data Dictionary .
|=--------------------------------------------------=|
The database structure from user's layer provides
database schema. Database schema specifies inner
structure organization, semantics and constraints for
data access. Entity-Relatioin (E-R) model provides
description of database schema structure. It's
constructed of the basic elements called entities.
And it show the relation between it.
Entity is the image of thing or object from the
real world distinct from other object. Entities
are described in the database by specification.
Entity specification includes it's character and
set of attributes expressing their attributes.
Relation express relationship between entities. Basic
principle of data extraction from database consists
of search for given relation. If it is too much
information in the relation it's common to implement
mechanism to speed the access to it. In the case of
relation databases the principle is implemented on
the physical layer by index usage.
Database schema is set of definitions given in the
a definition language. The result of the commands in
the data definition language is te set of information
stored in special file called data dictionary. Data
dictionary includes metadata. That can be described
as information on data stored in the database. Data
dictionary is used by DBMS (Database Management
System) before it will start to manipulate or define
database structure. Data manipulation is the set of
transaction including selecting, inserting, deleting
or updating of the information stored in the data
base. Data manipulation Language allows user access
and data manipulation corresponding to it's inner
structure.
