The ANSI/SPARC Architecture of a Database Environment

It allows independent customised user views AMERICAN NATIONAL STANDARDS INSTITUTE (ANSI) STANDARDS PLANNING AND REQUIREMENTS COMMITTEE (SPARC) It allows independent customised user views It hides the physical storage details from users The database administrator should be able to change the database storage structures without affecting the users’ views The internal structure of the database should be unaffected by changes to the physical aspects of the storage e.g changeover to a new disk The database administrator should be able to change the conceptual or global structure of the database without affecting the users

Schema and instance database schema database instance the description of the database is called the database schema or intension; specified at the creation of the database not expected to change very often database instance the raw data that populates a database at a particular moment in time is called a database instance of the extension of the database

Schema and instance - example

The three ANSI/SPARC levels of abstraction

EXTERNAL LEVEL It represents the user’s view of the database It describes that part of the database that is relevant to a particular user May provide different representations of the same data e.g some may view dates in the form (day/month/year) or (year/month/day) Views may include derived or calculated data

CONCEPTUAL LEVEL Aka the community view of the database It represents the following: all entities, their attributes and relationships; the constraints on the data; security and integrity information It supports each external view in that any data available to a user must be contained in or derivable from the conceptual level

INTERNAL LEVEL It describes how the data is stored in the database in terms of particular data structures and file organizations Is concerned with: allocating storage space for data and indexes; describing the forms that records will take when stored; record placement; data compression and encryption techniques Below the internal level is the physical level which is managed by the OS under the direction of the DBMS and deals with the mechanics of physically storing data on a device such as a disk

External / conceptual example

Conceptual / internal - example

Data independence Refers to the ability to change the schema at a lower level and the changes do not affect the schema at a higher level; of 2 types: physical the immunity of conceptual schema to changes at the internal level logical the immunity of external schema to changes at the conceptual level