Databases

Databases

Cutajar & Cutajar

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Conventional File Approach

Each section keeps a separate file for storing information

Example: Attendance and Results files

STUDENT ATTENDANCE INFORMATION

Surname Name ID Address Father Class ABSENT

Zammit John 217787M 7 Main Street, Zebbug Charles IT7B 23

Cutajar Mary 323187G 9 St. Martin Street, Zejtun George IT4C 1

Piscopo Natalie 787887M 9, B. Zammit Street, Attard Peter II2F 78

STUDENT RESULTS INFORMATION

Surname Name ID Address Class Subject Mark

Zammit John 217787M 7 Main Street, Zebbug IT7B English 45

Cutajar Mary 323187G 9 St. Martin Street, Zejtun IT4C Maths 90

Piscopo Natalie 787887M 9, B. Zammit Street, Attard II2F Korean 12

Duplication of data

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Problems in Conventional File Systems

The computer-based subsystems, being self-contained, do not represent the way in which organizations really work

Systems communicate with each other outside the computer

Possibly data inconsistencies

Data duplication

Data non-shareable

Overall view of company to help decision-making process is almost next to impossible because of the large amount of work involved. Information from different departments has to be collated before appropriate statistics could be obtained - much duplication of data is likely, creating unnecessary maintenance, risk of inconsistencies and wastage of storage space.

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Example

Consider a customer order

Customer order(input)

ORDER PROCESSING

SYSTEM

Invoice(output)

Customer (debtor)

data

SALES ACCOUNTING SYSTEM

(debtor’s control)

Statements & confirmation of payments

Required stock (lacking)

PURCHASING

SYSTEM Stock order

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Integrated File Systems

To overcome some of the problems with conventional file systems, integrated file systems are employed. Here, the data is pooled into a set of interlocking and interdependent files, which are accessible by a number of different users (and applications).

Process

File A

File B

File C

E.g.:

Stock file

E.g.:

Order file

…

User 1

User 2

User 3

…

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Problems with integrated file systems.

When a transaction enters the system, ALL the appropriate files are updated.

Most integrated file systems in use are usually tailor-made to meet the requirements of particular organizations.

Data duplication problem still remains

The task of maintaining the data is shared between the programs that access and maintain the data and therefore lacks proper central control.

Programs are still dependant on the nature of the data stored and vice-versa.

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DataBase Management System

Databases represent a radically different approach to solving the problems discussed.

Definition: A DATABASE is a single organized collection of structured data stored with a minimum of duplication of data items so as to provide a consistent and controlled pool of data. This data is common to all users of the system, but is independent of the programs that use it.

(Alternative definition: A DATABASE is collection of non-redundant data shareable between different application systems)

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DBMS

It is essentially a computerized record-keeping system.

PROCESS

USER 1

USER 2

USER 3

D

B

M

S

DATABASE

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Database Access

Note that, the DBMS is the only medium through which any user may access the data in the database.

USER 3

USER 4

USER 5

USER 1

USER 2

DBMS

DataBase

Here it is assumed that the data is contained in a single database. In practice there is often reason for the data to be split across several distinct databases.

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Computer System

Database management systems are available on machines ranging from small microcomputers to the largest mainframes. The facilities provided are to some extent determined by the size and power of the underlying machine.

On large machines one would usually find a multi-user system.

On small microcomputers it is more usual to have a single user database system.

Multi-User SystemSingle-User System

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Hardware Requirements

With regards to hardware requirements, databases demand large secondary storage facilities, and other demands depending on the circumstances.

Distributed System

Direct access storage e.g. disc packs, and so on.

On-line storage i.e. computer can access a given record without human intervention

In a multi-user system, users can access the database at the same time

For distributed databases, a fast, reliable communications network would be a requirement.

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Database Environment

In a Database Environment there are three classes of users:

Application Programmer Responsible for writing programs that use the database

End User May access database via an application program

Database Administrator (DBA)Has central responsibility of the system.

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Application Programmer

Responsible for writing programs that use the database (by issuing appropriate requests to the DBMS)

Batch applications – on-line applications

Function of programs is to support an end user

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End User

May access database via an application program or use the interface provided as an integral part of the system by which the user is able to issue high level commands to the DBMS.

Typical functions include:

Data Entry

Issue of Reports

Queries

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Database Administrator (DBA)

Co-ordinates database design

Loads the database

Controls access to the database (ensures privacy and security)

Establishes backup and recovery procedures

Controls changes to the database

Selects and maintains database management software

Meets the users to resolve problems and determine changing requirements

The DBA has central responsibility of the system.

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Advantages of Databases vs Traditional Methods

Compactness : no need for voluminous paper files

Speed : the machine can retrieve and change data much faster than any human

Less drudgery : much of the sheer tedium of maintaining files by hand is eliminated

Currency : accurate up-to-date information is available on demand at any time.

Faster and cheaper development of new applications

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In case of a multi-user environment

The database system provides the enterprise with centralized control of its data

Redundancy can be reduced (more usually controlled rather than totally eliminated

Inconsistency can be avoided Data can be shared Standards can be enforced: to enable data interchange; standards may

include corporate, installation, industrial, national, international Security restrictions can be applied: ensuring access to database through

proper channels; provision of security checks on access Integrity can be maintained: ensuring that the data in the database is

accurate Conflicting requirements can be balanced: provide overall service that is

‘best for the enterprise’ Provision of data independence i.e. if application programs are revised,

these changes do not effect the database itself and vice-versa.

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Disadvantages of databases vs traditional methods

High initial investment in hardware, software and training

Generality that a DBMS provides for defining and processing data

Overhead for providing security, concurrency control, recovery, and integrity functions

(additional problems may arise if the database designers and DBA do not properly design the database or if the database systems applications are not implemented properly)

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The Database Management System

The database management system (DBMS) is a complex software system (system software), which constructs, expands and maintains the database.

Main objective of DBMS

The DBMS handles all requests from the users for access to the database hence shielding users from hardware level details.

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Functions of a DBMS

Allocates storage to data Maintains indices so that any required data can be retrieved, allowing for

separate items of data in the base can be cross-referenced Maintains data in the database by providing facilities for adding new

records, deleting records, amending records and adding new sets of records (data files)

Provides an interface with user programs: Programmer need not be familiar with the structure of the database, and write programs in different programming languages

File processing facilities: Process a complete file (serial/sequential), process required records (selective sequential/random) and retrieve individual records

Provides security of data: Protects data against unauthorized access, corruption and providing recovery and restart facilities after a hardware or software failure

Keeps statistics of the use made of the data, allowing redundant data to be removed and Frequently used data to be kept in the most accessible form

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Data Modeling

In designing a database for an enterprise, initially one has to identify and state what data needs to be stored – the data requirements.

Example: Stock control application-

Each stock item has a stock number, a description, price, quantity in stock, reorder level and supplier

Each item has only one supplier

From this statement of data requirements a conceptual data model is produced.

OPERATIONAL DATA = ENTITIES + RELATIONSHIPS

E.g. suppliers E.g. suppliers supplies a certain kind of part – hence link between supplier and part entities

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The Conceptual Data Model

A conceptual data model describes how the data elements in the system are to be grouped. Three terms are used in building a picture of the data requirements. An entity is a thing of interest to an organization about which data is to

be stored. - Example: stock, supplier,

An attribute is a property or characteristic of an entity. - Example: stock item description

A relationship is a link or association between entities.- Example: link between stock items and suppliers: - one supplier may supply many stock items

This conceptual/logical model is created without any regard to what type of database system will eventually be used to implement it.Example: the entity stock and its attributes are expressed as follows:

STOCK(part_number, description, price, quantity, reorder level, supplier)

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Types of Relationships

Each relationship has a name (e.g. supplies) and a degree, which may be any of:

one-to-one

one-to-many (or many-to-one)

many-to-many

Example :- the relationship between supplier and stock:

Name : supplies (as indicated)

Degree: one-to-many

(a supplier supplies many stock items)

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Other Examples

E.g. 1: One-to-one: a specialist is assigned to a single ward

(Given that a specialist has one ward of patients under his care)

E.g. 2: One-to-many: a hospital ward occupied by many patients (but a patient is assigned to only one ward).

E.g. 3: Many-to-many: a student may study several courses, each course has many students enrolled on it.

Exercise: Identify the relationship between these entities:

stock item and supplier

CD and song

Borrower and book

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Entity-Relationship Diagrams(E-R Diagrams)

An entity-relationship diagram is a diagrammatic way of representing the relationships between the entities in a database. To show the relationship between two entities, both the degree andthe name of the relationship need to be specified.

An E-R diagram illustrates the entities and their relationships for a specific system.

is occupied by

SPECIALIST WARD

WARD PATIENTS

STUDENT COURSE

studies

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Example 1

The data requirements for a hospital in-patient system are defined as follows: A hospital is organized into a number of wards. Each ward has a ward

number and a name recorded, along with a number of beds in that ward. Each ward is staffed by nurses. Nurses have their staff number and name recorded, and are assigned to a single ward.

Each patient in the hospital has a patient identification number, and their name, address and date of birth are recorded. Each patient is under the care of a single consultant and is assigned to a single ward. Each consultant is responsible for a number of patients. Consultants have their staff number, name and specialisation recorded.

(i) State four entities for the hospital in-patient system and suggest an identifier for each of these entities.

(ii) Draw an entity-relationship diagram to show the relationship between the entities.

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Solution 1

ENTITY NAME

Ward Ward Number

Nurse Staff Number

Patient Patient Identification Number

Consultant Staff Number

staffed by visitsholds

PATIENTWARDNURSE CONSULTANT

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Example 2

Drawing an E-R diagram to represent this situation:

CUSTOMER ORDER

ORDER ITEM

CUSTOMER ITEM

All the relationship

pairs

is for

sends receives

ORDER

CUSTOMER

ITEM

In a mail order system, a customer may place an order for several items. Over a period of time, the same customer may place several orders.