CHAPTER 2 Database System Concepts and Architecture Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 1 Outline Data Models and Their Categories Schemas Instances and States Three Schema Architecture Data Independence DBMS Languages and Interfaces Database System Utilities and Tools Centralized and Client Server Architectures Classification of DBMSs Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 2 Data Models and Schemas One fundamental characteristic of the database approach is that it provides some level of data abstraction Data abstraction generally refers to the suppression of details of data organization and storage Data abstraction highlight the essential features for an improved understanding of data Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 3 Data Models Data model database Collection of concepts that describe the structure of a Basic operations Specify retrievals and updates on the database Dynamic aspect or behavior of a database application Allows the database designer to specify a set of valid operations user defined operations on database objects Copyright 2017 Ramez Elmasri and Shamkant B Navathe 4 Categories of Data Models 1 Conceptual high level semantic data models Provide concepts that are close to the way many users perceive data Also called entity based or object based data models Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 5 Categories of Data Models continue 2 Implementation representational data models Used by many commercial DBMS implementations e g relational data models used in many commercial systems Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 6 Categories of Data Models continue 3 Physical low level internal data models Provide concepts that describe details of how data is stored in the computer These are usually specified in an ad hoc manner through DBMS design and administration manuals Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 7 Categories of Data Models continue 4 Self Describing Data Models Combine the description of data with the data values Examples include XML key value stores and some NoSQL systems Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 8 Schemas versus Instances Database Schema The description of a database Includes descriptions of the database structure data types and the constraints on the database Schema Diagram An illustrative display of most aspects of a database schema Schema Construct A component of the schema or an object within the schema e g STUDENT COURSE Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 9 Schemas versus Instances Database State The actual data stored in a database at a particular moment in time This includes the collection of all the data in the database Also called database instance or occurrence or snapshot The term instance is also applied to individual database components e g record instance table instance entity instance Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 10 Database Schema vs Database State Refers to the content of a database at a moment Database State in time Initial Database State Refers to the database state when it is initially loaded into the system Valid State A state that satisfies the structure and constraints of the database Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 11 Database Schema vs Database State continued Distinction The database schema changes very infrequently The database state changes every time the database is updated Schema is also called intension State is also called extension Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 12 Three Schema Architecture Proposed to support DBMS characteristics of Program data independence Support of multiple views of the data Not explicitly used in commercial DBMS products but has been useful in explaining database system organization Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 13 Three Schema Architecture Defines DBMS schemas at three levels Internal schema at the internal level to describe physical storage structures and access paths e g indexes Typically uses a physical data model Conceptual schema at the conceptual level to describe the structure and constraints for the whole database for a community of users Uses a conceptual or an implementation data model External schemas at the external level to describe the various user views Usually uses the same data model as the conceptual schema Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 14 Three Schema Architecture Mappings among schema levels are needed to transform requests and data Programs refer to an external schema and are mapped by the DBMS to the internal schema for execution Data extracted from the internal DBMS level is reformatted to match the user s external view e g formatting the results of an SQL query for the display in a Web page Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 15 Data Independence Logical Data Independence The capacity to change the conceptual schema without having to change the external schemas and their associated application programs Physical Data Independence The capacity to change the internal schema without having to change the conceptual schema For example the internal schema may be changed when certain file structures are reorganized or new indexes are created to improve database performance Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 16 Data Independence continued When a schema at a lower level is changed only the mappings between this schema and higher level schemas need to be changed in a DBMS that fully supports data independence The higher level schemas themselves are unchanged Hence the application programs need not be changed since they refer to the external schemas Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 17 DBMS Languages Data Definition Language DDL Data Manipulation Language DML Copyright 2017 Ramez Elmasri and Shamkant B Navathe Slide 2 18 DBMS Languages Data Definition Language DDL Used by the DBA and database designers to specify the conceptual schema of a database In many DBMSs the DDL is also used to define internal and external schemas views In some DBMSs separate storage definition language SDL and view definition language VDL are used to define internal and external schemas SDL is typically realized via DBMS commands provided to the DBA and database designers Copyright 2017 Ramez Elmasri