Chap 1 Intro to Db

8/13/2019 Chap 1 Intro to Db

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MIST811: Week 3 1

Week 3: Introduction to Databases

The database is the underlying

framework of the information system.

Relational Database Management

Systems (DBMSs) is the predominant

system for business applications at

present.

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3.1 Introduction

A database is a collection of related data.

The Database Management System (DBMS) is thesoftware that manages and controls access to thedatabase.

A database application is a program that interactswith the database at some point of its execution.

The database system is a collection of applicationprograms that interact with the database along withthe DBMS and the database itself.

More accurate definitions will be provided later.


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Examples

Purchases from a supermarket

Bar code reader to scan each purchase.

Bar code reader is linked to an application program thatuses the bar code to find out the price of the item from aproduct database.

This program reduces the number of such items in stockand displays the price on the cash register.

If the reorder level falls below a specified threshold, thedatabase system may automatically place an order toobtain more stocks of that item.

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Examples ctd

Purchases using your credit card

Assistant normally checks you have sufficient credit left tomake the purchase. This can be done on the telephone ormay be done automatically by a card reader linked to acomputer system.

There is a database somewhere that contains informationabout the purchases you have made on your credit card.

To check your credit there is a database applicationprogram that uses your credit card number to check thatthe price of the goods you wish to buy together with thesum of the purchases you have already made this billingperiod is within your credit limit.


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Credit card example ctd

After confirmation of the purchase the details of

the purchase are added to this database.

The application program also accesses the

database to check that the credit card is not on a

list of stolen credit cards before authorising the

purchase.

There are other application programs to send outmonthly statements to each credit card holder

and to credit accounts when payment is received.

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Examples ctd

Booking a holiday using a travel agency

When you make enquiries about a holiday, the travel agentmay access several databases containing holiday andflight details.

When you book the holiday, the database system has tomake all the necessary booking arrangements.

The system has to ensure that two different agents do notbook the same holiday or overbook the seats on a flight.

The travel agent may have another, usually separate,database for invoicing.


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MIST811: Week 3 9

When does the manual f iling system

break down?

The manual filing system breaks downwhen we have to cross-reference orprocess information in the files.

Example:A typical real estate agentsoffice might have a separate file for

each property for sale or rent, eachpotential buyer and renter, and eachmember of staff.

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Real Estate Example ctd

How easy would it be to answer these questions using this set up?

What three-bedroom properties do you have for sale with a gardenand garage?

What flats do you have for rent within three kilometres of the citycentre?

What is the average rent for a two-bedroom flat? What is the total annual salary bill for staff?

How does last months turnover compare with the projected figurefor this month?

What is the expected monthly turnover for next financial year?


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Real Estate Example ctd

The file-based system was developed in

response to the needs of industry for more

efficient data access. A decentralised approach

was taken, where each department, with the

assistance of Data Processing (DP) staff,

stored and controlled its own data.

We will illustrate this using the DreamHomeexample which will be outlined on the following

slides.

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DreamHome Example

The Sales Department is responsible

for selling and renting property.

The form that landlord has to fill outbefore property is marketed for rent is

given on next slide.

This gives details of rental property as

well as owner (landlord) details.


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Figure 3.1

Sales Department

forms:

(a) Property for

Rent Details form

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DreamHome Example ctd

- Sales Department

also handles

enquiries from

clients (renters)

Figure 3.1 ctd

(b) Client Details

form.


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With assistance of Data Processing (DP)

Department, Sales Department creates an

information system to handle renting the

property

This consists of three files containing property, owner,

and client details (see next slide)

For simplicity details relating to staff members, branchoffices and business owners are omitted.

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Figure 3.2 Sales files used

PropertyForRent

PrivateOwner


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Figure 3.2 Sales files used ctd

PrivateOwner

Client

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Contracts Department is responsible for

rental agreements.

Whenever a client agrees to rent a

property a form is filled out by one of the

Sales staff (see next slide) which is

passed to the Contracts Department which

allocates a lease number and completes

the payment and rental period details.


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Figure 3.3

Lease

Details

form

used by

Contracts

Dept.

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With assistance from the Data Processing

(DP) Department the Contracts

Department creates an information system

to handle lease agreements. This consists of three files containing lease,

property, and client details (see next slide)

The data is similar to that held by the Sales

Department.


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Figure 3.4

Lease file

PropertyForRent file

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Figure 3.4 ctd

Client file

Lease file


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Figure 3.5 illustrates the situation.

Each department accesses their own filesthrough applications programs written speciallyfor them.

Each set of application programs handles dataentry, file maintenance, and the generation of afixed set of specific reports.

The physical structure and storage of the datafiles and records are defined in the applicationcode.

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Figure 3.5 File-based processing

Sales Files

PropertForRent (propertyNo, street, city, postcode, type, rooms,rent, ownerNo)

PrivateOwner(ownerNo, fName, lName, address, telNo)

Client (clientNo, fName, lName, address, telNo, prefType,maxRent)


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Figure 3.5 ctd

Contract Files

Lease (leaseNo, propertyNo, clientNo, rent, paymentMethod,deposit, paid, rentStart, rentFinish, duration)

PropertyForRent (propertyNo, street, city, postcode, rent)

Client (clientNo, fName, lName, address, telNo)

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There is a significant amount of

duplication of data in the two

departments.

This duplication of data is generally

true of file-based systems.


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Terminology used in file-based

systems

A file is simply a collection of records,

which contain logically related data.

Each record contains a logically

connected set of one or more fields,

where each field represents some

characteristics of the real-world objectthat is being modelled.

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3.2.2 Limitations of the File-Based

Approach

Separation and isolation of data

It is more difficult to access data that

should be available when the data isisolated in separate files.

The difficulty is compounded if we

required data from more than two files.


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Limitations of file-based systems ctd

Duplication of data

Uncontrolled duplication of data is undesirable

It is wasteful. It costs time and money to enterthe data more than once.

It takes up additional storage space which hascosts attached. Often duplication can be avoided

by sharing files. Duplication can lead to loss of data integrity; the

data is no longer consistent.

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Data dependence

The physical structure and storage of

the data files and records are definedin the application code. Making

changes to an existing structure is

difficult.


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In other organisations there was aproliferation of files and applicationprograms.

Eventually the DP Department with its currentresources could not handle all the work.

The pressure put on DP staff resulted inprograms that were inadequate or inefficient inmeeting the demand of the users, had limiteddocumentation, and maintenance that wasdifficult.

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Certain types of data functionality were

often sacrificed:

no prevision for security or integrity,

In the event of hardware or software failure

recovery was limited or non-existent,

no provision for shared access, access to

files was restricted to one user at a time.


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3.3 Database Approach

The limitations of the file-based approach

can be attributed to two factors:

1. The definition of the data is embedded in the

applications program, rather than being stored

separately and independently;

2. There is no control over the access and

manipulation of data beyond that imposed bythe applications programs.

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The database

The database and the Database ManagementSystem (DBMS) were developed to overcome theselimitations.

DEFN: The database is a shared collection of

logically related data, and a description of this data,designed to meet the information needs of anorganisation.

The database is a single, possibly large, repositoryof data that can be used simultaneously by manydepartments and users, All data items are integratedwith a minimum amount of duplication.


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The database ctd

The database is a shared corporate resource.

The database holds not only the organisationsoperational data but also a description of the data.For this reason, a database is also defined as a self-describing collection of integrated records. Thedescription of the data is known as the systemcatalog (or data dictionary or metadata the`data about the data). It is the self-describing natureof a database that provides program-dataindependence.

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The database ctd

The database approach separates the structure ofthe data from the application programs and stores itin the database. If the new data structures areadded or existing structures are modified then the

application programs are unaffected, provided theydo not directly depend upon what has beenmodified.

If we add a new field or record or create a new file, existingapplications are unaffected.

If we remove a field from a file that an application programuses, then that application program is affected by thischange and must be modified accordingly.


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Some more definitions

An entity is a distinct object (a person,place, thing, concept, or event) in theorganisation that is to represented in thedatabase.

An attribute is a property that describessome aspect of the object that we wish to

record. A relationship is an association between

entities.

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DreamHome Case Study

Figure 3.6 Example Entity Relationship (ER) Diagram for

part of the DreamHome


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DreamHome Case Study ctd

Figure 3.6 consists of:

Six entities (rectangles): Branch, Staff,PropertyForRent, Client, PrivateOwner, andLease;

Seven relationships (the names adjacent to thelines): Has, Offers, Oversees, Views, Owns,LeasedBy, and Holds;

Six attributes, one for each entity: branchNo,staffNo, propertyNo, clientNo, ownerNo, andleaseNo.

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DreamHome Case Study ctd

The database represents the entities,

the attributes, and the logical

relationships between the entities.

That is, the database holds data that islogically related.


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3.3.2 The Database Management

System (DBMS)

DEFN: The Database ManagementSystem (DBMS) is a software systemthat enables users to define, create,maintain, and control access to thedatabase.

The DBMS is the software thatinteracts with the users applicationprograms and the database.

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DBMS ctd

A DBMS usually provides the followingfacilities

It allows users to define the database,usually through a Data DefinitionLanguage (DDL). The DDL allowsusers to specify the data types andstructures and the constraints on thedata to be stored in the database.


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DBMS ctd

It allows users to insert, update, delete, and

retrieve data from the database, usually

through a Data Manipulation Language(DML). Having a central repository for all

data and data descriptions allows the DML

to provide a general inquiry facility to this

data, called a query language. The mostcommon query language is the Structured

Query Language (SQL).

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DBMS ctd

It provides access to the database. Itmay provide

A security system which preventsunauthorized users accessing thedatabase;

An integrity system which maintainsconsistency of stored data;


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DBMS ctd

A concurrency system which allows

shared access to the database;

A recovery system which restores the

database to a previous consistent state

following a hardware or software failure;

A user-accessible catalogue whichcontains descriptions of the data in the

database.

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DBMS ctd

DEFN: TheApplication Program is a computerprogram that interacts with the database byissuing an appropriate request (typically an SQLstatement) to the DBMS.

Users interact with the database through anumber of application programs that are usedto create and maintain the database and togenerate information. The application programsmay be written in some programming languageor in some higher-level fourth-generationlanguage.


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Figure 3.7

Pearson Education Limited 1995, 2005

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Views

DEFN:A view is some subset of the database. Itallows users to see the data the way they want tosee it.

Benefits of views:

Provide a level of security. Can exclude data thatsome users should not see.

Provide a mechanism to customize theappearance of the database,

Can present a consistent, unchanging picture ofthe structure of the database even if theunderlying database is changed.


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3.3.4 Components of the DBMS

Environment

There are five major components:

hardware, software, data,

procedures, and people.

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Hardware

Needed for DBMS and the applications to run on.

Can range from a single personal computer, to a singlemainframe, to a network of computers.

Depends on the organizations requirements and the DBMSused.

A DBMS requires a minimum amount of main memory anddisk space to run, but this minimum may not give acceptableperformance.

The frontend is the part of the DBMS that interfaces with theuser. This is called client-serverarchitecture: the backend isthe server and the frontends are the clients.


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Software

Made up of DBMS software and the

application programs, together with

operating system, including network

software if the DBMS is being used over a

network.

Application programs may be written using

a fourth-generation language like SQL

embedded in a third generation language.

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Data

From the end-users point of view data is themost important component of the DBMSenvironment.

Data is the bridge between the machinecomponents and human components.

The database contains both the operational dataand the metadata (data about data).

The structure of the database is called theschema.


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Data ctd

Figure 3.7 on earlier slide is a schemaconsisting of four files (or tables):PropertyForRent, PrivateOwner, Client, Lease.

PropertyForRent table has 8 fields or attributes:propertyNo, street, city, postcode, type (the propertytype), rooms (the number of rooms), rent (the monthlyrent), ownerNo.

ownerNo attribute models the relationship betweenPropertyForRent and PrivateOwner (the owner Ownsthe property for rent), see Figure 3.6.

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Procedures

Procedures are instructions and rules that governthe design and use of the database.

Documented procedures on how to run the systemare required by the users of the system and the staff

that manage the database. Procedures may consist of instructions on how to:

log on to the DBMS;

use a particular DBMS facility or application program;

start and stop the DBMS;

change the structure of a table.


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3.4 History of Database Management

Systems

First-generation (mid 1960s)

Hierarchical model (e.g. Information ManagementSystem) and network model by IDS (Integrated DataStore).

Second-generation

Relational model proposed by E.F. Codd in 1970. SQL isthe standard language for for relational DBMSs.

Third-generation Object-Oriented

Object-Relational

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3.5.1 Advantages of DBMSs

Control of data redundancy

Data consistency

More information from the same amount of data

Sharing of data

Improved data integrity

Improved security

Enforcement of standards

Economy of scale


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3.5.1 Advantages of DBMSs ctd

Balance of conflicting requirements

Improved data accessibility andresponsiveness

Increased productivity

Improved maintenance through dataindependence

Increased concurrency

Improved backup and recovery services

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3.5.2 Disadvantages of DBMSs

Complexity

Size

Cost of DBMSs Additional hardware costs

Cost of conversion

Performance

High impact of failure


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3.6 The Relational Model

The Relational Database Management System(RDBMS) is the dominant data-processing softwareused today.

This software is the second generation of DBMSs and isbased on the relational model of E.F. Codd (1970).

Codd was trained as a mathematician. Set theory andpredicate logic mainly underpin his model.

In the relational model, all data is logically structured

within relations (tables). Each relation has a name and is made up of named

attributes (columns) of data.

Each tuple (row) contains one value per attribute.

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3.6.1 Terminology

Relation:A relation is a table with columns and rows.

At tr ibute: An attribute is a named column of a relation.

Domain:A domain is the set of allowable values for one ormore attributes.

Tuple:A tuple is a row of a relation.

Degree: The degree of a relation is the number ofattributes it contains.

Cardinality: The cardinality of a relation is the number oftuples it contains.

Relational database:A collection of normalized* relationswith distinct relation names.

*(More on this in a later lecture.)


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Alternative terminology

FieldColumnAttribute

RecordRowTuple

FileTableRelation

Alternative 2Alternat ive 1Formal terms

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3.6.2 Relational Keys

Superkey:An attribute, or set of attributes, that

uniquely identifies a tuple within a relation.

Candidate key:A superkey such that no proper

subset is a superkey within the relation.Primary key: The candidate key that is selected to

identify tuples uniquely within the relation.

Foreign key:An attribute, or set of attributes,

within one relation that matches the candidate

key of some (possibly the same) relation.


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Representing Relational Database

Schemas

The common convention is to give the name ofthe relation followed by the attribute names inparenthesis. The primary key is usuallyunderlined.

(Q2 of the tutorial exercises provides anexample.)

The conceptual model, or conceptual schema, is

the set of all such schemas for the database.(Q2 of the tutorial exercises provides anexample.)

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Integrity Constraints

Null: Represents a value for an attribute that iscurrently unknown or is not applicable for thistuple.

Base relation:A named relation corresponding to

an entity in the conceptual schema, whosetuples are physically stored in the database.

Entity Integrity: In a base relation, no attribute ofthe primary key can be null.

Referential Integrity: If a foreign key exists in arelation, either the foreign key value must matcha candidate key value of some tuple in its homerelation or the foreign key must be wholly null.


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3.6.3 Views

View: The dynamic result of one or more relationaloperations operating on the base relations to produceanother relation. A view is a virtual relation that does notnecessarily exist in the database but can be producedupon request by a particular user, at the time of request.

Views provide a powerful and flexible securitymechanism by hiding parts of the database from certainusers.

Views permit users to access data in a way that iscustomised to their needs.

Views can simplify complex operations on the baserelations.

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Tutorial Exercises for Week 3

The Wellmeadows Hospital CaseStudy is from DATABASE SYSTEMS

A Practical Approach to Design,

Implementation, and Management.,Thomas Connolly & Carolyn Begg(2005), pp. 1260-1267,AddisonWesley.

This will be handed out in the lecture.


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Tutorial Exercises Week 3 ctd

1. Study the Wellmeadows Hospital Case Studyhand out.

a) How would a file based approach beimplemented?

b) In what ways would a DBMS help thisorganisation?

c) What data can you identify that needs to berepresented in the database?

d) What relationships exist between the data items?

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2. The following tables form part of a database held in a relationalDBMS:

Hotel (hotelNo, hotelName, city)

Room (roomNo, hotelNo, type, price)

Booking (hotelNo, guestNo, dateFrom, dateTo, roomNo)

Guest (guestNo, guestName, guestAddress)

where

Hotel contains hotel details and hotelNo is the primary key;

Room contains room details for each hotel and (roomNo,hotelNo)forms the primary key;

Booking contains details of bookings and(hotelNo,guestNo,dateFrom) forms the primary key;

Guest contains guest details and guestNo is the primary key.


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a) Identify the foreign keys in this schema. Explainhow the entity and referential integrity rules applyto these relations.

b) Produce some sample tables for these relationsthat observe the relational integrity rules. Suggestsome general constraints that would beappropriate for this schema.

HOMEWORK: Submit answers to 1a) 1b), 1c), 1d),2a), 2b) to ERIC by 6pm Monday 17 March2008.

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Readings

Chapter 1: An Overview of DatabaseManagement inAn Introduction to DatabaseSystems (8th edition), Addison-Wesley, 2004by C. J. Date. (QA76.9.D3 D3659 2004)

Chapter 1: Introduction to Databases inDATABASE SYSTEMS A Practical Approachto Design, Implementation, and Management(FOURTH EDITION), Addison Wesley, 2005 byThomas Connolly and Carolyn Begg.(QA76.9.D26 C66 2005)

Chap 1 Intro to Db

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