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Lecture 6

This document discusses database schemas and mapping between different schema types according to the ANSI-SPARC three-level architecture. It defines external, conceptual, and internal schemas and how the DBMS maps between them. Logical and physical data independence are achieved through this three-level architecture by making changes to lower levels independent from higher levels.

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Ahmad Hussain Shafi SE-3rd
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98 views11 pages

Lecture 6

This document discusses database schemas and mapping between different schema types according to the ANSI-SPARC three-level architecture. It defines external, conceptual, and internal schemas and how the DBMS maps between them. Logical and physical data independence are achieved through this three-level architecture by making changes to lower levels independent from higher levels.

Uploaded by

Ahmad Hussain Shafi SE-3rd
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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CSC241 Database Systems

Lecture # 6
By
Maria Gul

1
Schemas
• External Schemas
❖Also called subschemas.
❖Multiple schemas per database.
❖Corresponds to different views of data.
• Conceptual Schema
❖Describes all the entities, attributes, and
relationships together with integrity
constraints.
❖Only one schema per database.
Schemas
• Internal Schema
❖A complete description of the internal model,
containing the definitions of stored records, the
methods of representation, the data fields, and
the indexes and storage structures used.
❖Only one schema per database.
Mapping
• The DBMS is responsible for mapping between these three types
of schema:
• The DBMS must check that each external schema is derivable from
the conceptual schema, and it must use the information in the
conceptual schema to map between each external schema and the
internal schema
• Types of mappings
• Conceptual/Internal mapping
• External/Conceptual mapping
Conceptual/Internal Mapping
• Enables the DBMS to
❖Find the actual record or combination of
records in physical storage that constitute a
logical record in the conceptual schema,
❖Together with any constraints to be enforced
on the operations for that logical record.
❖ It also allows any differences in entity names,
attribute names, attribute order, data types,
and so on, to be resolved.
External/Conceptual
Mapping
• Enables the DBMS to
❖Map names in the user’s view on to the
relevant part of the conceptual schema.
7
Data Independence
• Outcome of three level architecture is data
independence.
• Change in lower level do not affect the
upper level.
• Type of data Independence
❖Logical Data Independence

❖Physical data Independence


8
Logical Data Independence
• Changes in conceptual model do not affect
external view.
• Logical data independence is the
Immunity/resistance of the external schema
to changes in the conceptual schema.
• Example
❖Adding new file
❖Adding new fields
❖Change the type of Attribute
❖Delete the Attribute 9
• Physical data Independence
• Changes in Internal model do not affect
conceptual model.
• Physical data independence is the
Immunity/resistance of the conceptual schema
to changes in the Internal schema.
• Example
❖Changing in file organization
❖Index Implementation
❖Using different storage devices

10
Data Independence and the ANSI-
SPARC Three-Level Architecture

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