Chapter 2 1

Chapter 2Database System Concepts

andArchitecture

Chapter 2 2

Data Models

• A data model is a description of the structure of a database. Data models generally, fall into 3 categories according to the level of the description.

Chapter 2 3

• High-level ( or conceptual) data models closely approximate the miniworld.

• Representational ( or, implementational ) data models are intermediate data models close to the miniworld but also reflecting the actual organization of data in the database.

• Low-level ( or, physical) data models describe details of physical storage, generally transparent to casual or parametric end users.

Chapter 2 4

• A data model is often specified by a database schema typically displayed in a schema diagram that consists of schema constructs.

Chapter 2 5

University Database SchemaDiagram

• fig 2.1

Chapter 2 6

Database State

• The actual data in a data base at a particular instant is the database state, which consists of a set of instances for each schema construct.

• Defining a database consists of specifying a schema to the DBMS. We then have a database in an empty state, with no data.

Chapter 2 7

• When data is first loaded, the database is in its initial state.

• Subsequently, each update creates another state. The DBMS must guarantee that each such state is a valid state that satisfies schema specifications.

• The schema is the intension, while a database state is the extension of the schema.

Chapter 2 8

ExternalView1

ExternalView1

Conceptual Schema

Internal Schema

External/High Level

Chapter 2 9

DBMS Languages

• DDL - Data Definition Language is used to define both schemas.

• VDL - View Definition Language is used to specify the mapping of the user views to the conceptual schema.

• DML - Data Manipulation Language is used to provide a means to manipulate the database. (I.e. insert, delete, query,etc)

Chapter 2 10

• Most DBMS’s combine the various capabilities of the DDL, VDL, DML and SDL into a single high-level DML (e.g. SQL relational database language.)

• Such DML’s can generally be entered interactively from the terminal or embedded in a high-level language such as C which acts as a host language.

Chapter 2 11

Common Approaches by Users

• Typically, sophisticated end users embed the DML .

• Casual end users usually access interactively through user-friendly interfaces created by too developers– Menu based interfaces– Graphical interfaces– Forms base interfaces

Chapter 2 12

• Parametric end users are provided with tailored interfaces for canned transactions that are designed by a systems analyst.

Chapter 2 13

Components of a DBMS

• fig 2.3

Chapter 2 14

Database System Utilities

• Loading

• Conversion

• Backup

• File Reorganization

• Performance monitoring

• Communication

Chapter 2 15

Database Classifications

• Single-user vs. Multiuser

• Centralized vs. Distributed

• Data Model– Relational– Network– Hierarchical– Object-oriented