Download Chapter 2 Database System Concepts and Architecture

Chapter 2
Database System Concepts
and
Architecture
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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.
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• 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.
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• A data model is often specified by a
database schema typically displayed in a
schema diagram that consists of schema
constructs.
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University Database Schema
Diagram
• fig 2.1
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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.
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• 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.
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External
/High Level
External
View1
External
View1
Conceptual Schema
Internal Schema
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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)
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• 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.
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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
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• Parametric end users are provided with
tailored interfaces for canned transactions
that are designed by a systems analyst.
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Components of a DBMS
• fig 2.3
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Database System Utilities
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Loading
Conversion
Backup
File Reorganization
Performance monitoring
Communication
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Database Classifications
• Single-user vs. Multiuser
• Centralized vs. Distributed
• Data Model
–
–
–
–
Relational
Network
Hierarchical
Object-oriented
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