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Database systems - Chapter 2

The document discusses the objectives and components of the ANSI-SPARC three-level database architecture. The architecture includes an external, conceptual, and internal level. The external level defines users' views, the conceptual level defines entity relationships and constraints, and the internal level defines physical storage. Mappings allow translation between levels. The architecture aims to provide logical and physical data independence so changes to one level do not affect others.

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Database Systems
Database Environment Chapter 2
Objectives of Three-Level Architecture  ANSI-SPARC Three Level Architecture  All users should be able to access same data but have a different customized view  A user’s view is immune to changes made in other views  Users should not need to know physical database storage details
Objectives of Three-Level Architecture..  DBA should be able to change database storage structures without affecting the users’ views  Internal structure of database should be unaffected by changes to physical aspects of storage  DBA should be able to change conceptual structure of database without affecting all users
ANSI-SPARC Three-Level Architecture
ANSI-SPARC Three-Level Architecture..  External Level  Users’ view of the database  Describes that part of database that is relevant to a particular user  Different views may have different representation of same data (e.g. different date formats, age derived from DOB etc.)
ANSI-SPARC Three-Level Architecture..  Conceptual Level  Community view of the database  Describes what data is stored in database and relationships among the data  Along with any constraints on data  Independent of any storage considerations i. all entities, their attributes, and their relationships; ii. the constraints on the data; iii. semantic information about the data security and integrity information.
ANSI-SPARC Three-Level Architecture..  Internal Level  storage space allocation for data and indexes;  record descriptions for storage (with stored sizes for data items); n  record placement; n  data compression and data encryption techniques.
ANSI-SPARC Three-Level Architecture..  Internal Level  Physical representation of the database on the computer  Describes how the data is stored in the database  physical implementation of the database to achieve optimal runtime performance and storage space utilization  Data structures and file organizations used to store data on storage devices  Interfaces with the operating system access methods to place the data on the storage devices, build the indexes, retrieve the data, and so on
Differences between Three Levels of ANSI-SPARC Architecture
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
Mappings  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
Instances  Database Schema  Description of database (also called intension)  Specified during design phase  Remain almost static  Database Instance  Data in the database at any particular point in time  Dynamic (changes with the time)  Also called an extension (or state) of database
Data Independence  Logical Data Independence  Refers to protection of external schemas to changes in conceptual schema  Conceptual schema changes (e.g. addition/removal of entities)  Should not require changes to external schema or rewrites of application programs
Data Independence  Physical Data Independence  Refers to immunity of conceptual schema to changes in the internal schema  Internal schema changes (e.g. using different file organizations, storage structures, storage devices etc.)  Should not require change to conceptual or external schemas
Data Independence and the ANSI-SPARC Three-Level Architecture
Summary  ANSI-SPARC three-level architecture  Schemas, mappings, and instances  Data independence