Databases MMG508

DB Properties  Definition of a database: “A database is a collection of interrelated data items that are managed as a single unit”  MS Access – all data is contained in a single file  ORACLE – all data is contained in a “collection” of physical files that are managed by an instance of their database software

DB Properties (Cont’d)  Some definitions Instance – a copy of the specific database software running in memory Definitions are very interesting – one term can mean different things between two vendors Database Object is “a named data structure that is stored in a database” Database Model is an organization methodology that is designed to emulate the real world

DB Properties (Cont’d)  Definitions (Cont’d) File – a collection of related records stored as a single unit by an operating system Using the term “File” for a database is inappropriate because these files do not contain:  Management by the DBMS  Layers of data abstraction  Physical data independence  Logical data independence

Four Major DB Properties  First – The Database Management System (DBMS)  Provides services to: Move data to and from physical files Manage usage of concurrent data between various users Managing transactions Support for a query language (SQL*Plus (Oracle), MYSQL, SQL, PL/SQL (Oracle)

Four Major DB Properties (Cont’d)  Procedures for Backup and Recovery of the database  Security

Four Major DB Properties (Cont’d)  Second - Layers of Data Abstraction Provision of User Views The Physical Layer  Data files that hold all the data for the database.  Most modern DBMS store the database in multiple files The Logical Layer  This is the logical layout of the database  For the most part, looks like an organization chart

Four Major DB Properties (Cont’d)  Layers of Data Abstraction (Cont’d) The External Layer  This is where users and application programs that access the database connect and issue queries against the database.  Third - Physical Data Independence The ability to make changes to the physical file structure without interrupting user actions

Four Major DB Properties (Cont’d)  Fourth - Logical Data Independence The ability to make changes to the logical db layer without disrupting existing users

Database Models  Flat files Files that are created and utilized by standard computer operations (See p. 8)  The Hierarchical Model This model was used in the first dbms’ (looks like an organization chart) (See pp )  The Network Model Similar to the hierarchical model

Database Models (Cont’d)  Relational Model Developed by Dr. E.F. Codd This was revolutionary work Let’s look at Figures 1-7/1-8 on pp. 14/15  Object-Oriented Model  Object-Relational Model

Why Focus on RDBMS?  As you text states: Definition, maintenance, and manipulation of data storage structures is easy Data is retrieved through simple ad hoc queries Data is well protected Well-established ANSI and ISO standards exist There are many vendors from which to choose Conversion between vendor implementations is relatively easy RDBMSs are mature and stable products

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