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Database System Architecture
The main objective of DBMS is to store and retrieve information efficiently; all the users
should be able to access same data. The designers use complex data structure to represent
the data, so that data can be efficiently stored and retrieved, but it is not necessary for the
users to know physical database storage details. The developers hide the complexity from
users through several levels of abstraction.
These different levels are:
i.
Physical Level
It is concerned with the physical storage of the information. It provides the internal
view of the actual physical storage of data. The physical level describes complex
low-level data structures in detail.
Logical Level
Logical level describes what data are stored in the database and what relationships
exist among those data. Logical level describes the entire database in terms of a
small number of simple structures. The implementation of simple structure of the
logical level may involve complex physical level structures; the user of the logical
level does not need to be aware of this complexity. Database administrator uses the
logical level of abstraction.
View Level
ii.
iii.
View level is the highest level of abstraction. It is the view that the individual user of
the database has. There can be many view level abstractions of the same data.
Data Model: A set of concepts to describe the structure of a database, and certain
constraints that the database should obey. By structure of a database it means the data
types, relationships, and constraints that should hold on the data
Categories of data models



Conceptual (high-level, semantic) data models: Provide concepts that are close to
the way many users perceive data. Conceptual data models use concepts such as
entities, attributes, and relationships.
Physical (low-level, internal) data models: Provide concepts that describe details
of how data is stored in the computer. Physical data models describe how data is
stored in the computer by representing information such as record formats, record
orderings, and access paths.
Implementation (representational) data models: Provide concepts that fall
between the above two, balancing user views with some computer storage details.
They include relational data model, object data models, legacy data models—the
network and hierarchical models
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CST_04204 Database principles
Database System Architecture
In any data model it is important to distinguish between the description of the database
and the database itself. The description of a database is called the database schema, which
is specified during database design and is not expected to change frequently however the
actual data in a database may change quite frequently.
Database Schema: The description of a database. It includes descriptions of the database
structure and the constraints that should hold on the database.
Three-Schema Architecture
The goal of the three-schema architecture, illustrated in Figure below, is to separate the
user applications and the physical database. In this architecture, schemas can be defined at
the following three levels



Internal schema at the internal level to describe physical storage structures and
access paths. Typically uses a physical data model.
Conceptual schema at the conceptual level to describe the structure and
constraints for the whole database for a community of users. Uses a conceptual or an
implementation data model.
External schemas at the external level to describe the various user views. Usually
uses the same data model as the conceptual level.
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Database System Architecture
Data Independence
Data independence can be defined as the capacity to change the schema at one level of a
database system without having to change the schema at the next higher level. We can
define two types of data independence


Logical Data Independence: The capacity to change the conceptual schema
without having to change the external schemas or application programs.
Physical Data Independence: The capacity to change the internal schema without
having to change the conceptual schema.
When a schema at a lower level is changed, only the mappings between this schema
and higher-level schemas need to be changed in a DBMS that fully supports data
independence. The higher-level schemas themselves are unchanged. Hence, the
application programs need not be changed since they refer to the external schemas.
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