1 Chapter 2 Database Environment

2 Objectives of Three-Level Architecture u All users should be able to access same data u User’s view immune to changes made in other views u Users need not know physical database storage details

3 Objectives of Three-Level Architecture u DBA can change database storage structures without affecting users’ views u Internal structure of database unaffected by changes to physical aspects of storage u DBA should be able to change conceptual structure of database without affecting all users

4 ANSI-SPARC Three-Level Architecture

5 u External Level –Users’ view of the database. –Describes that part of database that is relevant to a particular user. u Conceptual Level –Community view of the database. –Describes what data is stored in database and relationships among the data.

6 ANSI-SPARC Three-Level Architecture u Internal Level –Physical representation of the database on the computer. –Describes how the data is stored in the database.

7 Differences between Three Levels of ANSI- SPARC Architecture

8 Data Independence u Logical Data Independence –Refers to immunity 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

9 Data Independence u Physical Data Independence –Refers to immunity of conceptual schema to changes in internal schema –Internal schema changes (e.g. using different file organizations, storage structures/devices) –Should not require change to conceptual or external schemas

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

11 Database Languages u Data Definition Language (DDL) –Allows DBA or user to describe and name entities, attributes, and relationships –plus any associated integrity and security constraints

12 Database Languages u Data Manipulation Language (DML) –Provides basic data manipulation operations on data held in database u Procedural DML –allows user to tell system exactly how to manipulate data u Non-Procedural DML –allows user to state what data is needed rather than how it is to be retrieved –Fourth Generation Languages (4GLs)

13 Data Model Integrated collection of concepts for describing data, relationships between data, and constraints on the data in an organization. u Data Model comprises: –a structural part; –a manipulative part; –possibly a set of integrity rules.

14 Data Model u Purpose –To represent data in an understandable way u Categories of data models include: –Object-based –Record-based –Physical

15 Data Models u Object-Based Data Models –Entity-Relationship –Semantic –Functional –Object-Oriented u Record-Based Data Models –Relational Data Model –Network Data Model –Hierarchical Data Model u Physical Data Models

16 Relational Data Model

17 Network Data Model

18 Hierarchical Data Model

19 Functions of a DBMS u Data Storage, Retrieval, and Update u User-Accessible Catalog u Transaction Support u Concurrency Control Services u Recovery Services

20 Functions of a DBMS u Authorization Services u Support for Data Communication u Integrity Services u Services to Promote Data Independence u Utility Services

21 System Catalog u Repository of information (metadata) describing data in database u Fundamental components of DBMS u Typically stores: –names, types, and sizes of data items –constraints on the data –names of authorized users –data items accessible by user and type of access –usage statistics

22 Components of a DBMS

23 Components of Database Manager (DM)

24 Chapters Covered u Chapters 1 & 2 Today u Next week Chapters 4 & 5 u Assignment 1