lec-3 describing & storing data in database

7/30/2019 Lec-3 Describing & Storing Data in Database

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DESCRIBING AND STORINGDATA IN A DATABASE

Lecture 3


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DB Schema vs. DB State

Database Schema (intension)

description of the database

is specified during database design Database State (extension of the schema)

current state of the database

actual data instances (occurrences) in a DB

changes over time by update

initially, a database is empty state with no data

then, populate (load) the database with data


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Database Schema

Valid State

DBMS checks every state of the database

does it satisfy the structure and constraintsspecified in the schema?

Schema Diagram

for displaying database schema

schema construct: an object in the schema

no instance of records


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Database Schema (cont)

Meta-data

descriptions of the schema constructs and

constraints stored in the database catalog

Schema Evolution

Schema change prompted by the change of

application requirements


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2.1.2 Schemas, Instances and Database State

Database Schema (meta-data): The description of a database. Includes

descriptions of the database structure and the constraints that should holdon the database.

Schema Diagram: A diagrammatic display of (some aspects of ) a

database schema.

Database Instance: The actual data stored in a database at aparticular

moment in time. Also called database state ( or occurrence, snapshot)

The database schema changes very infrequently. The database state

changes every time the database is updated. Schema is also called

intension, whereas state is called extension.

Each schema construt has its own current set of instances.


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Figure 2.1 Schema diagram forUNIVERSITY database

schema construct

Known data:

name of record types, data items


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Figure 1.2

UNIVERSITY Database


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Database schema (e.g. relational):

Names and types of attributes

Addresses

Indexing

Statistics

Authorization rules to access data etc.

Data independence: separation of the physical and logical data

Particularly important for distributed systems

The mapping between them is provided by the schema


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Levels of Abstraction

External

schema 1

External

schema 1

External

schema 1

Conceptual schema

Physical schema

Disk


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External Schema Describes several views of the database based on the

database model. Several external schemas are possible for a single database.

Each view is based upon the user requirements.

Data access to be customized at a level of individual users orgroups of users

Each conceptual schema consist of collection of one or moreviews.

Example:

StdGrade(RegNo:Integer, Name:String,Sem:Integer, Grade:Char)


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Conceptual Schema Describes the stored data in terms of the data model of the

DBMS. This leads to conceptual database design.

There is only one Conceptual schema of Database.

DBMS maps from conceptual schema to physical schema

Example:

Student(RegNo:Integer, Name:String,

Sem:Integer, Branch:String)Faculty(Fid:Integer, FName:String, Salary:Float)

Course(CourseNo:Integer, CName:String,

Credit:Integere, Dept:String)

Section(SecId:Integer, CourseNo:Integer,

Sem:Integer, Year:Integer, Instructor:String)GradeReport(RegNo:Integer, SecId:Integer,

Grade:Char)


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Physical Schema

Describes the actual storage details of the relations described

in conceptual schema.

This leads to the physical database design.

Physical schema specifies additional storage devices

Describes how the relations described in conceptual schemaare stored on secondary storage devices such as disks and

tapes.


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Program-Data Independence

The data independence is the ability tochange the schema at one level of a databasesystem with out changing the schema at a

higher level. Logical data Independence

capacity to change the conceptual schemawithout having to change the externalschema, is called as the logical dataindependence. With out changing theapplication programs, one can change thelogical schema.
http://www.doag.de/orafaq/glossary/faqgloss.htmhttp://www.doag.de/orafaq/glossary/faqgloss.htm


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Example

Suppose the Faculty relation is modified

as:Faculty_Public(Fid:Integer, FName:String,Office:Integer)

Faculty_Private(Fid:Integer, Salary:Float)

Any view designed before this modificationcan still retrieve the data with little

modification (relation name) and obtainthe same answer.


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Physical data independenceCapacity to change the internal schema without

having to change the conceptual schema isknown as Physical data Independence.

There are occasions for changing the

internal structures for improvedperformance of the retrieval of data.

Any change introduced to the internal

schema or physical schema will not affectthe other schemas.

lec-3 describing & storing data in database

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