Chapter 2 DBMS

The Three-level ArchitectureI. External modelII. Logical or conceptual modelIII. Internal or physical model

Logical Data ModelsI. Semantic data modelII. Record based model

DATABASE ARCHITECTURE

THE THREE-LEVEL ARCHITECTURE

External model Logical Or Conceptual Model Internal Or Physical Model

THE THREE-LEVEL ARCHITECTURE

EXTERNAL LEVEL

• The external level interacts directly with the user (end users or application programs). It changes the data coming from the conceptual level to a format and view that is familiar to the users.

• Consists of many user models or views• Has external records - records seen by users• May include calculated or virtual data• Devide in external schemas (sub-schemas)• Used to create user interface

LOGICAL OR CONCEPTUAL MODEL

The conceptual level defines the logical view of the data.

• All entities, attributes, relationships represented• Includes all record types, data item types, relationships, semantic

information, security and integrity information• Described in logical schema

INTERNAL LEVEL

• The internal level determines where data is actually stored on the storage devices.

• The DBMS and Operating System View• Physical implementation level• Includes data structures, file organizations used by DBMS• Depends on what DBMS is used

WHAT IS DATA INDEPENDENCE

• Techniques that allow data to be changed without affecting the applications that process it.

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TYPES OF DATA INDEPENDENCE

• Logical Data Independence: • The capacity to change the conceptual schema without having to change the

external schemas and their associated application programs.

• Physical Data Independence:• The capacity to change the internal schema without having to change the

conceptual schema.• For example, the internal schema may be changed when certain file

structures are reorganized or new indexes are created to improve database performance

RECORD BASED MODELS

¤ In this model database is structured in fixed-format records of several types.

¤ They allow the designer to develop and specify the logical structure and provide some options for implementation of the design

¤ It describes the data at the conceptual and view levels both.

¤ Specify overall logical structure of the database

I. Hierarchical ModelII. Network ModelIII. Relational Model

Types:

HIERARCHICAL MODEL

¤ Oldest¤ Like a structure of a tree¤ Branches represent fields¤ Nodes represent record¤ Structure contains levels, or segments.¤ Higher layer is perceived as the parent of the segment

directly beneath it, which is called the child

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Example:

NETWORK MODEL

¤ Like hierarchical model¤ But may have many to many relation¤ It allows a record to have more than one parent¤ Replaces the hierarchical tree with a graph

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Example:

:- A student taking two classes:- A teacher teaches different classes:- An employee works for two departments

Examples:

RELATIONAL MODEL

¤ Proposed by E.F. Codd¤ Uses relations¤ Contains tables¤ Tables represent relationships ¤ Columns represent attributes¤ It’s better than others

STDID STDNAME MAJOR CREDITS

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Student - TableEnrollment - Table

Class - Table

C_ID STDID Grade

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