Download Key Relational and SQL Concepts (part 1 of 3)

Revision of Key Database Concepts
(Part 1 of 3)
Why Database Systems?
Data
One of the most important resources in ALL organisations.
Without information, hence data, how could we:
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Control manufacturing processes?
Process sales of goods?
Diagnose patients’ illnesses?
Forecast sales?
Run the University of Teesside?
Information
To be useful, information must be:
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Accurate
Timely
Relevant
Therefore, you need adequate facilities for:
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Storing (and verifying) data
Manipulating data
Extracting data
Traditional Approach
From the earliest days computers were used to store files of information.
Separate systems, ie separate files and programs, were developed for each
application, eg payroll files, personnel files, accounts files, etc.
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Modules
Data File
Module
System
Module
Descriptions
Student
Data File
Student
System
Student
Records
Room
Resource
System
Resource
Summary
Data File
Problems
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Inconsistency
Redundancy
Lack of integration and control
Solution?
The Database Approach
Instead of having separate files for separate applications, data are organised into a
single set of underlying files from which the applications draw the data that are
relevant to them.
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Module
System
Module
Data
Student
Data
Database
Management
System
Room
Data
Student
System
Resource
System
What is a Database?
“A shared collection of logically related data, and a description of this data, designed
to meet the information needs of an organisation.”
‘Database Systems’ by Connolly & Begg
Addison-Wesley, ISBN 0-201-70857-4
“A database system can be thought of as a computerised record-keeping system.
Such a system involves the data itself (stored in the database), hardware, software
and – most important! – users.
… Databases are integrated and (usually) shared; they are used to store persistent
data.”
‘An Introduction to Database Systems’ by
C J Date, Addison-Wesley, ISBN 0-201-38590-2
(7th Edition, 2000)
What is a Database Management System (DBMS)?
“A software system that enables users to define, create, maintain, and control access
to the database.”
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Provides the interface between the user and the data in the database.
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Allocates storage to data and maintains indices so that any required data can
be retrieved.
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Protects data against unauthorised access.
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Safeguards data against corruption.
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Provides recovery and restart facilities after a hardware or software failure.
Advantages of the Database Approach
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No unnecessary duplication of data.
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Greater consistency of data.
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Wider availability of data.
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Greater flexibility of use of data.
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Improved data integrity.
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Improved security.
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Improved backup and recovery services.
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Can change the data structure without altering associated programs.
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A database is dynamic: it can grow and change.
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Data management can be more consistent and systematic.
The Three Level ANSI-SPARC Architecture
In 1975 the ANSI Standards Planning and Requirements Committee proposed a
standard terminology and general architecture for database systems. The objective
is to separate each user’s view of the database from the way it is physically
represented.
3 levels or views of data within a database:
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External Level
The users’ view of the database. Also known as the applications view.
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Conceptual Level
The overall view of the database. Also known as the global view.
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Internal Level
The physical representation of the database on the computer. Also known as
the storage view.
Schemas
The overall description of the database is called the database schema. There are 3
different types of schema in the database.
External Schema
There are multiple external schemas (or subschemas), each one corresponding to a
different view of the data.
Conceptual Schema
There is one conceptual schema, which describes the data stored in the database,
the relationships and the integrity constraints.
Internal Schema
There is one internal schema, which describes how the data are stored in the
database and how they are accessed.
Mapping
Provides the translation between the schemas at different levels. The DBMS is
responsible for mapping between the 3 types of schema.
The DBMS must ensure that each external schema is derivable from the conceptual
schema.
The DBMS must use the information in the conceptual schema to map between each
external schema and the internal schema.
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Data Independence
A major objective for the 3-level architecture is to provide data independence, ie
upper levels must be unaffected by changes to lower levels.
There are 2 kinds of data independence:
Logical data independence refers to the immunity of the external schemas to
changes in the conceptual schema.
It should be possible to alter tables, columns or relationships without having to alter
existing external schemas or rewrite application programs (other than those that are
directly affected).
Physical data independence refers to the immunity of the conceptual schema to
changes in the internal schema.
It should be possible to alter file organizations, storage devices, indexes, etc, without
having to alter the conceptual or external schemas.
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The System Catalogue
The database schema is defined using a special language called a Data Definition
Language (DDL).
The result of the compilation of the DDL statements is a set of tables stored in special
files collectively called the system catalog.
This is a repository of meta-data (data about data), ie information describing the data
in the database, typically containing the name, description, source and usage
information for each data item.
The system catalog is also known as the data dictionary or the data directory.
Database Languages
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A Data Definition Language (DDL) is used to specify the data in the database.
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A Data Manipulation Language (DML) is used to access the data.
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A Data Control Language (DCL) is used to control access to the data.
Some databases have a combined DDL, DML and DCL (often called a Query
Language), eg SQL.
Types of Database
5 main logical structures (in terms of how data are organised, stored and
manipulated):
1.
2.
3.
4.
5.
Hierarchical
Network
Relational
Object-oriented
Object-relational
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