ppt on dbms day 3
TRANSCRIPT
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Data ModelData Model
y Data Model is a collection of concepts
that can be used to describe the structure
of a data base.
y It provides the necessary means to
achieve data abstraction.(Hiding details of
data storage that are not needed by most
database users.)
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Structure of databaseStructure of database
y Structure means data types, relationships,
and constraints that should hold for the
data.
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SchemaSchema
y The description of data base is called
database schema.
y Description of data at some level.
y Each level has its own schema
y It is specified during database design and
is not expected to change frequently.
y A displayed schema is called schema
diagram.
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InstanceInstance
y The data in the database at a particular
moment in time is called a database state
or snapshot.
y It is also called instance.
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3 level architecture of data base3 level architecture of data base
Concerned with three forms of schemas:
y Physical,
y
Conceptualy External.
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Physical Data level schemaPhysical Data level schema
y The physical schema describes details
of how data is stored: files, indices, etc. on
the random access disk system.
y It also typically describes the record
layout of files and type of files.
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Conceptual Data LevelConceptual Data Level
y Hides details of the physical level.
y In the relational model, the conceptual
schema presents data as a set of tables
y Physical schema can be changed without
changing application:
y DBMS must change mapping from
conceptual to physical.
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External Data LevelExternal Data Level
y An external schema specifies a view of
the data in terms of the conceptual level.
y It is tailored to the needs of a particular
category of users.
y Portions of stored data should not be
seen by some users and begins to
implement a level of security andsimplifies the view for these users
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Data IndependenceData Independence
y Capacity to change the schema at one
level of a database system without having
to change the schema at next higher level.
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Types of Data IndependenceTypes of Data Independence
y Logical Data base independence
y Physical database independence
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Logical database independenceLogical database independence
y It is the capacity to change the conceptual
schema without having to change external
schemas.
y We can add more records or data type in
database without affecting end user or
external schema.
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Physical data independencePhysical data independence
y It is the capacity to change the internal
schema( physical level) without having to
change the conceptual schema.
y Here changes made to storage media, or
new structure of table is created will not
affect external and logical schema.
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DBMS LanguageDBMS Language
y Data definition language
y Data manipulation language
y Data control language