1 chapter 9 database management systems. 2 objectives for chapter 9 zproblems in the flat-file...

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Chapter 9

Database Management Systems

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Objectives for Chapter 9 Problems in the flat-file approach to data management

Why these gave rise to the database concept Relationships among the elements of the database

environment Characteristics of the relational database model Three stages in database design including

Conceptual design Logical design Physical design

Anomalies caused by unnormalized databases and the need for data normalization

Features of distributed databases Considerations in deciding on a particular database

configuration

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Flat-File Versus Database Environments

Computer processing involves two components: data and instructions (programs).

Conceptually, there are two methods for designing the interface between program instructions and data: File-oriented processing: A specific data file was

created for each application Data-oriented processing: Creates a single data

repository to support numerous applications Disadvantages of file-oriented processing include

redundant data and programs and varying formats for storing the redundant data.

The format for similar fields may vary because the programmer used inconsistent field formats.

Flat-File Environment

Program 1

Program 2

Program 3

A,B,C

X,B,Y

L,B,M

User 2Transactions

User 1Transactions

User 3Transactions

Data

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Data Redundancy & Flat-File Problems

Data Storage - creates excessive storage costs of paper documents and/or magnetic form

Data Updating - any changes or additions must be performed multiple times

Currency of Information - potential problem of failing to update all affected files

Task-Data Dependency - user’s inability to obtain additional information as his or her needs change

Program 1

Program 2

Program 3

User 2Transactions

User 1Transactions

User 3Transactions

Database

DBMS

A,B,C,X,Y,L,M

Database Approach

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Advantages of the Database Approach

Data sharing/centralize database resolves flat-file problems:

No data redundancy - Data is stored only once, eliminating data redundancy and reducing storage costs.

Single update - Because data is in only one place, it requires only a single update procedure, reducing the time and cost of keeping the database current.

Current values - A change to the database made by any user yields current data values for all other users.

Task-data independence - As users’ information needs expand beyond their immediate domain, the new needs can be more easily satisfied than under the flat-file approach.

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Disadvantages of the Database Approach

Can be costly to implement additional hardware, software, storage, and

network resources are requiredCan only run in certain operating

environments may make it unsuitable for some system

configurationsBecause it is so different from

the file-oriented approach, the database approach requires training users may be inertia or resistance

Elements of the Database Approach

System DevelopmentProcess

Database Administrator

USERS

DBMS

HostOperatingSystem

PhysicalDatabase

UserPrograms

UserPrograms

UserPrograms

Applications

DataDefinitionLanguage

DataManipulationLanguage

QueryLanguage

User Queries

Transactions

Transactions

Transactions

Sys

tem

Req

ue

sts

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DBMS FeaturesUser Programs - make the presence of the

DBMS transparent to the userDirect Query - allows authorized users to

access data without programmingApplication Development - user created

applicationsBackup and Recovery - copies databaseDatabase Usage Reporting - captures

statistics on database usage (who, when, etc.)Database Access - authorizes access to

sections of the database

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Internal Controls and DBMS

The purpose of the DBMS is to provide controlled access to the database.

The DBMS is a special software system programmed to know which data elements each user is authorized to access and deny unauthorized requests of data.

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Data Definition Language (DDL)

DDL is a programming language used to define the database to the DBMS.

The DDL identifies the names and the relationship of all data elements, records, and files that constitute the database.

Viewing Levels: internal view - physical arrangement of

records (1) conceptual view - representation of

database (1) user view - the portion of the database

each user views (many)

External User Views

Conceptual Model

Internal Model

Physical Database

ANSI Model

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Data Manipulation Language (DML)

DML is the proprietary programming language that a particular DBMS uses to retrieve, process, and store data.

Entire user programs may be written in the DML, or selected DML commands can be inserted into universal programs, such as COBOL and FORTRAN.

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Query Language

The query capability permits end users and professional programmers to access data in the database without the need for conventional programs.

IBM’s Structured Query Language (SQL) is a fourth-generation language that has emerged as the standard query language.

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Three Steps in Designing a Database

Prepare the conceptual model Identify the entities Identify the relationships between the entities Prepare the ER diagram

Specify the logical design Select the logical database model (relational) Transform the conceptual data model using a logical

database model

Implement the physical design Physical structures Access methods

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Phase 1

Prepare the Conceptual Model

Draw an ERD to capture the process.

ER-Diagram Symbols

Entity Relationship Attribute

Primary Key

ER-Diagram Symbols

Example of a Relationship LinkingTwo Entities

CUSTOMER places ORDER

Name Number

Order Number Item #

1 M

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An Entity

...is an individual object concept event

...may be a specific tangible object intangible object

Entity Class is a collection of entities with similar attributes.

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Attributes

A property of an entity that we choose to record (of interest to an organization).CUSTOMER (entity) PRODUCT (entity)customer # product #name descriptionaddress finishtelephone no. pricebalance qty. on hand

Sales-person

Car

Customer Order

Vendor Inventory

Assigned

Places

Supply

Entity Relationship Entity

1

M

M M

1

1

Cardinalities

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ER-Diagram using REA Model

Inventory Line items Sales Party to Salesperson

Pays for

Cash CollectionsIncreases

Cash

Made toCustomer

Cashier

Receivedfrom

Received by

M

1

M

M

M

M

M

M

M

M

1

1

1

1

R E A

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Phase 2

Specify the Logical Design

Create relational tables.

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Logical Data Structures

A particular method used to organize records in a database is called the database’s structure.

The objective is to develop this structure efficiently so that data can be accessed quickly and easily.

Four types of structures are: hierarchical (AKA the tree structure) network relational object-oriented

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The Relational Model

The relational model portrays data in the form of two dimensional tables: relation - the database table attributes (data elements) - form

columns tuples (records) - form rows data - the intersection of rows and

columns

RESTRICT - filtering out rows, such as the purple

PROJECT - filtering out columns,such as the purple

X1 X1

X2 X2

X3 X3

Y1

Y1

Y1 Y1

Y1

Y2 Y2 Y2

Y3

Z1 Z1

Z2 Z2

Z3 Z1

JOIN

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Properly Designed Relational Tables

No repeating values - All occurrences at the intersection of a row and column are a single value.

The attribute values in any column must all be of the same class.

Each column in a given table must be uniquely named.

Each row in the table must be unique in at least one attribute, which is the primary key.

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Relational Model Data Linkages (>1 table)

No explicit pointers are present. The data are viewed as a collection of independent tables.

Relations are formed by an attribute that is common to both tables in the relation.

Assignment of foreign keys: if 1 to 1 association, either of the table’s primary

keys may be the foreign key. if 1 to many association, the primary key on one of

the sides is embedded as the foreign key on the other side.

if many to many association, may embed foreign keys or create a separate linking table.

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Three Types of Anomalies

Insertion Anomaly: A new item cannot be added to the table until at least one entity uses a particular attribute item.

Deletion Anomaly: If an attribute item used by only one entity is deleted, all information about that attribute item is lost.

Update Anomaly: A modification on an attribute must be made in each of the rows in which the attribute appears.

Anomalies can be corrected by creating relational tables.

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Advantages of Relational Tables

Removes all three anomaliesVarious items of interest (customers,

inventory, sales) are stored in separate tables.

Space is used efficiently.Very flexible. Users can form ad hoc

relationships.

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The Normalization Process

A process which systematically splits unnormalized complex tables into smaller tables that meet two conditions: all nonkey (secondary) attributes in the

table are dependent on the primary key all nonkey attributes are independent of the

other nonkey attributesWhen unnormalized tables are split and

reduced to third normal form, they must then be linked together by foreign keys.

Steps in Normalization

Table withrepeating groups

First normalform 1NF

Second normalform 2NF

Third normalform 3NF

Higher normalforms

Removerepeating

groups

Remove partial

dependencies

Removetransitive

dependencies

Removeremaininganomalies

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Accountants and Data Normalization

The update anomaly can generate conflicting and obsolete database values.

The insertion anomaly can result in unrecorded transactions and incomplete audit trails.

The deletion anomaly can cause the loss of accounting records and the destruction of audit trails.

Accountants should have an understanding of the data normalization process and be able to determine whether a database is properly normalized.

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Phase 3

Implement the Physical Design

Decide about software and hardware.

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Physical Database Design

Transition from theoretical to physical aspects of database IS IT

Decisions about software and hardware Implementation

Populate the database with data Produce physical user views (multiple)

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Data Structures

allow records to be located, stored, and retrieved and allow movement through the database. Two components: The organization of a file is the physical

arrangement of records. The access method is the technique

used to locate records and to navigate through the database.

Distributed Data Processing

Site C Site BSite A

Centralized Database

Central Site

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Distributed Data Processing

DP is organized around several information processing units (IPUs) distributed throughout the organization and placed under the control of the end users.

DDP does NOT mean Decentralization! IPUs are connected to

one another and coordinated.

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Potential Advantages of DDP

Cost reductions in hardware and data entry tasks

Improved cost control responsibilityImproved user satisfaction since

control is closer to the user levelBackup of data can be improved

through the use of multiple data storage sites

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Potential Disadvantages of DDP

Loss of controlMismanagement of organization-wide

resourcesHardware and software incompatibilityRedundant tasks and dataConsolidating incompatible tasksDifficulty attracting qualified

personnelLack of standards

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The data is retained in a central location.

Remote IPUs send requests for data.Central site services the needs of

the remote IPUs.The actual processing of the data is

performed at the remote IPU.

Centralized Databases in DDP Environment

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Data Currency

Occurs in DDP with a centralized database

During transaction processing, the data will temporarily be inconsistent as a record is being read and updated.

Database lockout procedures are necessary to keep IPUs from reading inconsistent data and from writing over a transaction being written by another IPU.

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Distributed Databases: Partitioning

Splits the central database into segments that are distributed to their primary users

Advantages: users’ control is increased by having data

stored at local sites transaction processing response time is

improved the volume of transmitted data between IPUs is

reduced reduces the potential data loss from a

disaster

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The Deadlock Phenomenon

Especially a problem with partitioned databases

Occurs when multiple sites lock each other out of data that they are currently using One site needs data locked by another site.

Special software is needed to analyze and resolve conflicts. Transactions may be terminated and have

to be restarted.

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The Deadlock Phenomenon

A,BE, F

C,D

Locked A, waiting for C

Locked C, waiting for E

Locked E, waiting for A

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Distributed Databases: Replication

The duplication of the entire database for multiple IPUs

This method is effective for situations with a high degree of data sharing, but no primary user, and supports read-only queries.

The data traffic between sites is reduced considerably.

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Concurrency Problems and Control Issues

Database concurrency is the presence of complete and accurate data at all IPU sites. With replicated databases, maintaining current data at all locations is a difficult task.

Time stamping may be used to serialize transactions and to prevent and resolve any potential conflicts created by updating data at various IPUs.