DeSiaMore 1 Database Management Systems (DBMS) DeSiamore .

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Database Management Systems (DBMS)

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File Processing……….

Advantages of Sequential file processing It uses magnetic tape, the least expensive method

of secondary storage. It is the most efficient form of organisation when

the entire of file or most of it is processed at once Transaction file and old master file together act as

a back-up, it can be used to create the new master file when existing one damaged or destroyed



Disadvantages .. Time factor –the time it takes to access a

particular record may be too long for many applications

The entire file most be accessed and a new master file created , even if only one record requires maintenance or updating.



Direct File Organisation The data can be organised in such a way that they are

scattered throughout the disk This form of organisation that supports direct access also

referred to as random access The records can be accessed nearly instantaneously and in

any order When a record accessed, a record can be read or updated

and when this process competed , then the system is free to respond to another request



Direct processing requires either magnetic disk or optical disk and cannot use magnetic tape

Direct access systems do not search the entire file rather, they move direct or nearly directly to the required record, to do this the system must have some way to determine where a particular record is stored

Example, in figure 4, data are entered directly into the system through a terminal that is in contact with the CPU of the central computer, the system locates the specific record in the master file and then updates it.



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TerminalCPU

1 U

Direct Access Storage Device (DASD)Figure 4, Direct File Processing –Records are accessed directly



Direct file processing…. There are several strategies which are used to find record..

Relative addressing Hashing (randomising) Indexing

Relative addressing Simplest method of finding a record A record’s primary key is associated with a specific

physical storage location On retrieval process, the user enters the Key and the disk

operating system associates this key with the appropriate location on the disk



Relative addressing…. Relative addressing loses its appeal when the record key

cannot be made to match the physical location Hashing also known as Randomising

Method for determining the physical location of a record. The record key is processed mathematically and another

number is computed that represents the location where the record will be stored

Record keys are transformed into storage addresses and by using an arithmetic procedures called randomising or hashing algorithm



The task of this process is to take a set of records keys and find a formula to map them into set of disk storage location Identifiers

On retrieval process, user needs to retrieve the record once its key is entered and the hashing routine is used to determine where the record can be found on storage disks.

Major difficulty with the hashing procedure is due to that some addresses will never get generated whilst two or more record keys produce identical disk address or synonyms or Collisions



Indexing It uses a primary index which associates a primary key

with the physical location in which a record is stored.

Advantages of direct file organisation Data can be accessed directly and quickly Primary and secondary indexes can be used to search data

in many ways Files can still be processed sequentially using secondary

index Centrally maintained data can be kept up-date



Disadvantages of direct file processing The use of an index lowers the computer system’s

efficiency The hardware must be expensive for these

systems because all data must be stored on disks There will be no backup data if a file destroyed,

the files are updated directly and no transaction files are maintained on system



Summary on direct file organisation The choice of file organisation and the methods

used for direct access depend on the five characteristics File volatility File activity File query needs File size Data currency


Data Processing

Data processing comprises the following; Capturing of data Storing of data Updating and retrieving of data and information

Data Management Data management is the arrangement of all data and

information with an organization It also refers to the methods of physical storage and

retrieval of data on a disk or other storage devices


Data Management..

Data management involves the following, Data administration The standards of defining data The way in which people perceive and use data in

their day-to-day activities


Data Independence

Data Independence Data independence allows a database to be

structurally changed , it means data can be; Added and deleted or data attributes altered with

minimum disruption to the existing system This implies that application programs are not required

to have detailed knowledge of the records layout, it means when a record layout is changed like fields added, deleted, changed in size then fewer application programs or none would be changed


Data Independence….

There are two distinct levels of data independence; Logical data independence Physical data independence

Logical data independence Insulates application programs from logical

operations such as combining two records into one or splitting an existing record into two or more records


Data independence….

Physical data independence Indicates that the physical storage structures or

devices used for storing data could be changed, this happens without needing a change in the record structure or application programs


Database Administration Roles Database administration –Centrally

controlling the database Implemented by a person or group of persons

under the supervision of a knowledgeable person called Administrator, this person known as Database Administrator (DBA)

DBA is responsible for supervising the creation, modification and maintenance of the database

The DBA controls the database structure and sets up the definition for physical as well as logical implementation of the database.


Database Administration Roles.. Implementing Security Features,

DBA maintains the integrity of a database DBA maintains that the database is not accessible by

unauthorised users, DBA is responsible for granting permission to use the

database and stores the profile of each user The user profile can be used by the DBA to verify that a

particular user is allowed to access and perform a given operation on database within the limited time frame


Database Administration Roles… Measures Against Data Loss

DBA is responsible for defining procedures to recover data from failures –human natural, or hardware malfunctioning with minimum loss

DBMS Users The users of a database can be classified

depending on their degree of expertise or their mode of interactions with the DBMS.


Database Administration Roles… DBMS Users…..

The user can be classified as.. Naïve Users Online Users Application Programmers BDAs

Naïve Users Are those users who need not be aware of the presence of

the database system or any other system supporting their usage

Example, the users of an Automatic Teller Machine fall in this category.


Database Administration Roles… Naïve Users…

The user is instructed through each step of a transaction, user respond by pressing a coded key or numeric value

Operations that can be performed by Naïve user are very limited and affect on precise portion of the database

Naïve users are ‘end users of the database who work through a menu driven application program where the type and range of response is always indicated to the user’

( Jain et al., 2002)


Database Administration Roles… Online Users

Are those users who may communicate with the database directly via an online terminal or indirectly via a user interface and application program

These users are aware of the presence of the database system and may have acquired a certain amount of expertise with the limited interaction they are permitted with a database


Database Administration Roles… Application programmers

Professional programmers, who are responsible for developing application programs or user interface

Database Administrator DBA is a knowledgeable person who is

responsible for the physical design and management of the database


Data Dictionary

A Data Dictionary is a database about databases, it holds the following information about each data element in the databases; Name Type Range of values Source Access authorization Indicates which application programs use the data.

A data dictionary may be a stand-alone information system used for management and documentation purposes, or it may be an integral part of the database management system.

Data dictionary is used to actually control the database operations, data integrity and accuracy


Data Dictionary…..

Importance of Data Dictionary It provides the name of a data element, its

description, and data structure in which may be found

Provides great assistance in producing a report of where a data element is used in all programs that mention it

It is possible to search for a data name, provided keywords that describe that name


Database Processing

Data Items

Database

Relationships

Constraints

SchemaFigure 1, Components of a Database


Database processing…..

What is a Database? A collection of data designed to be used by different people, Organised in such a way that a computer program can quickly

select desired piece of information A database consists of four elements;

Data Relationships Constraints Schema

Data Binary computer representations of stored logical entities They are distinct piece of information usually formatted in a

special way. The term data is often used to distinguish binary (machine-

readable) information from textual (human-readable) information.


Database Processing….

Example, some applications make a distinction between data files (files that contain binary data) and text files (files that contain ASCII data)

In database management systems, data files are the files that store the database information whereas other –index files and data dictionaries, stores administrative information known as metadata



Relationships Relationships represent a correspondence between the

various data elements Constraints

Are predicates that define correct database states. Schema

Describes the organisation of data and relationships within the database.

Defines various views of the database for the use of the system components of the database management system and for the application’s security as in figure 2



Schema… It separates the physical aspects of data storage from the

logical aspects of the representation As in figure 2,

The internal schema defines how and where data are organised in physical data storage

The conceptual schema defines the stored data structures in terms of the database model used.

The external schema defines a view/s of the database for particular users. In this case the database management system provides services for accessing the database whilst maintaining the required correctness and consistency of the stored data



External Schema 1

Physical Database

External Schema 2 External Schema N

Global Conceptual Schema

Internal Schema

……….

Figure 2, Organisation of a Database



Why a Database Why should an organisation have an integrated database

to store its operational data? Deficiencies of pre-database information processing

include (but not limit to) the following… Data inconsistency Lack data integrity Data repetition or redundancy Interdependence between programs and data files Lack of foolproof data security mechanisms Lack of coordination across applications using common data Non-uniform back-up and recovery methods Encoded data



The advantage of having the data in a database are; Redundancy can be reduced –having a centralised database

redundancy or multiple copies of the same data can be reduced Inconsistency can be avoided –this depends on data redundancy,

which means when the same data is duplicated and changes are made at one site, which is not propagated to the other site, then it gives rise to inconsistency. So if the redundancy is removed chances of having inconsistent data is also removed

Data can be shared –the existing application can share data in a database

Standards can be enforced –with the central control of the database, the DBA can enforce standards

Security restrictions can be applied –the DBA can define authorisation checks to be carried out wherever access to sensitive data is attempted.



Integrity can be maintained –integrity means that the data in the database is accurate. Centralised control of the data helps in permitting the administrator to define integrity constraints to the data in the database

Conflicting requirement can be balanced –database designers can be able to create database that is the best for the organisation by knowing the overall requirements.



Characteristics of Data in a Database Shared –a data in a database are shared among different users and

applications Persistence –data in a database exist permanently in the sense the

data can live beyond the scope of the process that created Validity/Integrity/ Correctness –data should be correct with respect to

the real world entity that they represent Security –data should be protected from unauthorised access Consistency –whenever more that one data element in a database

represents related real-world values, the values should be consistent with respect to the relationship

Non-Redundancy –no two data items in a database should represent the same real-world entity

Independence –the three levels in the schema (internal, conceptual and external) should be independent of each other so that changes in the schema at one level should not affect the other levels


Types of Database Language

There are three types of database languages DDL ( Data Definition Language) DML (Data Manipulation Language) DCL (Data Control Language)


Types of Database Language… DDL

Used to define data and their relationships to other types of data

Used to formulate schema-level concepts Mainly used to create files, databases, data

dictionaries, and tables within databases. Defines the format or schema of the database


Data Definition Language…

It allows specification of following information about each tables The schema of each table The integrity constraints The set of values associated with each attribute The security and authorization information for each table The physical storage structure of each table on disk

The SQL commands that are used to create database objects are known as Data Definition Language or DDL


Types of Database Language…… DML

DML is a language which deals with the processing or manipulation of various database objects

It provides for the program interface to open and close database, find records in files, navigate through the records, add new records and change or delete existing records

To formulate changes to be effected in a database instance

The SQL commands that are used to manipulate data within database objects are called Data Manipulation Language or DML


Types of Database Language…… DCL

DCL is a language which used to improve security features and thus prevents unauthorised access to data in the database

Security is provided by granting or revoking privileges on a user

Privileges determines whether or not a user can execute a given command or a command can be executed on specific groups of data

The SQL commands that are used to control the behaviour of database objects are called Data Control Language or DCL


Any QuestionsAny Questions


Database Management Systems (DBMS) Outline

Introduction to DBMS Database Architecture Database Management System Why DBMS Types of DBMS

DeSiaMore 1 Database Management Systems (DBMS) DeSiamore .

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