The ANSI/SPARC Architecture aka the 3 Level Architecture

AMERICAN NATIONAL STANDARDS INSTITUTE (ANSI) STANDARDS PLANNING AND REQUIREMENTS COMMITTEE (SPARC) It allows independent customised user views; The Three Level Architecture has the aim of enabling users to access the same data but with a personalised view of it; Each user should be able to access the same data, but have a different customized view of the data. These should be independent: changes to one view should not affect others. It hides the physical storage details from users; The distancing of the internal level from the external level means that users do not need to know how the data is physically stored in the database. This level separation also allows the Database Administrator (DBA) to change the database storage structures without affecting the users' views. 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

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 It excludes irrelevant data as well as data which the user is not authorised to access. 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 The conceptual level is a way of describing what data is stored within the whole database and how the data is inter-related but it does not specify how the data is physically stored. 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

CONCEPTUAL LEVEL continued..... The database administrator will have to be conversant with this layer, because most of his operations are carried out on it. Only a database administrator is allowed to modify or structure this level.

INTERNAL LEVEL The internal level involves how the database is physically represented on the computer system. It describes how the data is actually stored in the database and on the computer hardware. It describes how the data is stored in the database in terms of particular data structures and file organizations The internal level is the most technical of the three levels. However, the internal level view is still abstract meaning even if it shows how the data is stored physically, it will not show how the database software operates on it. 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

INTERNAL LEVEL continued..... 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

TYPES OF SCHEMAS There are three different types of schema corresponding to the three levels in the ANSI-SPARC architecture. The punggung or external schemas which describe the different external views of the data and there may be many external schemas for a given database. The conceptual schema describes all the data items and relationships between them, together with integrity constraints (later). There is only one conceptual schema per database. The internal schema at the lowest level contains definitions of the stored records, the methods of representation, the data fields, and indexes. There is only one internal schema per database.

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

REVIEW QUESTIONS With the aid of appropriate examples, explain the difference between a database instance and a database schema Describe the 3 levels of abstraction that were identified by the American National Standards Institute Special Planning and Requirements Committee c) What are the advantages of implementing the 3 level architecture in database design? d) Define the term data independence and identify the 2 types of data independence that exists in relation to the ANSI-SPARC Architecture e) Describe the types of schemas that exist in a database complying with the 3 levels ANSI-SPARC architecture