Concepts of Database

8/8/2019 Concepts of Database

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Concepts of database


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Data bases

A database can be defined as acollection of related data items stored inan organised way so that specific items

can be selected and retrieved quickly Centrally controlled, integrated collection

of logically organized data.

Data elements should be in specific placesand bear specific relationships to eachother.


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Advantages over conventional

file system In conventional systems data are

scattered over a large number of files with

no proper integration, logical co-ordinations or centralized control.

The data and programs are notindependent so for each application a new

program may have to be written

Time consuming and costly


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The Objectives of a DBMS

1. Mass storage of Relevant Data

The Whole data is logically organise,

integrated14 and controlled from a centralpoint.


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2. Data Access made easy

In conventional systems it is not easy.

Bank manager wants customers with

amt 50000 and above. Bank manager wants customers of age

less than 40.

So such environment does not allow one to

retrieve the needed data in a convenient andefficient manner

So better methods are needed to be developed


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3. Elimination of data redundancy andinconsistency

Data redundancy- duplication of data storing of data more than once

Redundancy leads to inconsistency eg: Name and address stored in personal

file as well as in salary file. Address change will lead to inconsistency

In DBMS care should be taken to avoid suchredundant information

Stored only once so no inconsistency


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4. Simultaneous use ofDatabases

The system should be made available

to multiple users at the same time toimprove the overall performance andobtain faster performance

Eg: Railway ticker booking


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5. Data integration

In conventional systems data is scattered invarious files in various formats

QUANTITY in numeric oralphabetic so making new application will betdifficult.

DBMS ensures that data type and width etc are

explicitly defined. It also ensures some consistency constraints

such as numeric data: minimum balance etc.


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6. Security of data base

Database is a important asset. So security isimportant

PAYROLL PERSON,

CUSOMER

DBMS ensures that only authorised persons can accessauthorised potions of the database.

Password control, accession levels etc.

Protections of data against accidental or deliberatedisclosure to unauthorised persons and unauthorisedmodification or destruction of data is calleddatasecurity


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7.Standards can be enforced -

Since all access to the database must bethrough DBMS, so standards are easier to

enforce.

Standards may relate to the naming of data,format of data, structure of the data etc.

Standardizing stored data formats is usuallydesirable for the purpose of data interchange ormigration between systems


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8. Backup and Recovery

Back up copy is made regularly

Bring the database to a statethat existed before failure.


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DBMS and its structure,

DBMS can be defined as a system whichhas a set of rules and methods that allows for

definition, creation, retrieval, updation,maintenance and protection of the database

DBMS (Database Management System) acts asan interface between the user and the database.

The user requests the DBMS to perform variousoperations (insert, delete, update and retrieval)on the database.


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The components ofDBMS perform these requestedoperations on the database and provide necessary datato the users.

Data base is a program or collection of program betweenDatabase and User.

- assist in manipulating database

- protect database from the user

Dbase, Clipper, Foxpro, Ms Access, Oracle, Sybase,

Ingress, Informix, MS-SQL server, DB2 Etc. Database systems are partitioned into modules for

different functions. Some functions (e.g. file systems)may be provided by the operating system.


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Components ofDBMS File managermanages allocation of disk space and data

structures used to represent information on disk.

Database manager: The interface between low-level data andapplication programs and queries.

Query processortranslates statements in a query language into

low-level instructions the database manager understands. (Mayalso attempt to find an equivalent but more efficient form.)

DML precompilerconverts DML statements embedded in anapplication program to normal procedure calls in a hostlanguage. The precompiler interacts with the query processor.

DDL compilerconverts DDL statements to a set of tables

containing metadata stored in a data dictionary.


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In addition, several data structures are requiredfor physical system implementation:

Data files: store the database itself.

Data dictionary: stores information about thestructure of the database. It is used heavily. Greatemphasis should be placed on developing a gooddesign and efficient implementation of the dictionary.

Indices: provide fast access to data items holdingparticular values.

Components ofDBMS


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Instances andSchemes

Databases change over time.

The information in a database at a particular point in timeis called an instance of the database.

The overall design of the database is called thedatabase scheme.

Analogy with programming languages: Data type definition - scheme

Value of a variable - instance

There are several schemes, corresponding to levels ofabstraction: Physical scheme

Conceptual scheme


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DATABASE LANGUAGES

ADBMS is a software package that carries out manydifferent tasks including the provision of facilities toenable the user to access and modify information in thedatabase.

The database is an intermediate link between thephysical database, computer and the operating systemand the users. To provide the various facilities todifferent types of users, a DBMS normally provides oneor more specialized programming languages called

database languages. Database languages come in different forms. They are: -

1. Data Description Language (DDL)

2. Data Manipulation Language (DML)


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Data Definition Language (DDL)

As the name suggests, this language is used to definethe various types of data in the database and theirrelationship with each other.

Used to specify a database scheme as a set of

definitions expressed in a DDL DDL statements are compiled, resulting in a set of tablesstored in a special file called a data dictionary ordatadirectory.

The data directory contains metadata (data about data)

The storage structure and access methods used by thedatabase system are specified by a set of definitions in aspecial type ofDDL called a data storage anddefinition language

basic idea: hide implementation details of the databaseschemes from the users


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The basic functions performed

by DDL are

Create tables, files, databases and datadictionaries.

Specify the storage structure of each tableon disk.

Integrity constraints on various tables.

Security and authorization information of

each table. Specify the structure of each table.

Overall design of the Database.


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Data Manipulation Language (DML) A language that enables users to access or manipulate data

(retrieve, insert, update, delete) as organized by a certain DataModel is called the Data Manipulation Language (DML).

Data Manipulation is: retrieval of information from the database insertion of new information into the database deletion of information in the database

modification of information in the database ADML is a language which enables users to access and manipulate

data. The goal is to provide efficient human interaction with the system. There are two types ofDML:

procedural: the user specifies whatdata is needed and how to get it

nonprocedural: the user only specifies whatdata is needed Easier for user May not generate code as efficient as that produced by procedural

languages

A query language is a portion of a DML involving informationretrieval only. The terms DML and query language are often usedsynonymously.


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Database Manager

The database manageris a program module whichprovides the interface between the low-level data storedin the database and the application programs andqueries submitted to the system.

Databases typically require lots of storage space(gigabytes). This must be stored on disks. Data is movedbetween disk and main memory (MM) as needed.

The goal of the database system isto simplify and facilitate access to data. Performance isimportant. Views provide simplification.


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So the database manager module is responsible for Interaction with the file manager: Storing raw data on disk

using the file system usually provided by a conventionaloperating system. The database manager must translate

DML statements into low-level file system commands (forstoring, retrieving and updating data in the database).

Integrity enforcement: Checking that updates in thedatabase do not violate consistency constraints (e.g. nobank account balance below $25)

Security enforcement: Ensuring that users only haveaccess to information they are permitted to see Backup and recovery: Detecting failures due to power

failure, disk crash, software errors, etc., and restoring thedatabase to its state before the failure

Concurrency control:P

reserving data consistency whenthere are concurrent users.

Some small database systems may miss some ofthese features, resulting in simpler databasemanagers. These features are necessary on larger

systems.


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Database Administrator

The

database a

dministratorisa person having central control over data and

programs accessing that data. Duties of thedatabase administrator include:

Scheme definition: the creation of the originaldatabase scheme. This involves writing a set ofdefinitions in a DDL (data storage and definitionlanguage), compiled by the DDL compiler into a setof tables stored in the data dictionary.

Storage structure and access methoddefinition: writing a set of definitions translated by

the data storage and definition language compiler



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Scheme and physical organization modification: writing a setof definitions used by the DDL compiler to generatemodifications to appropriate internal system tables (e.g. datadictionary). This is done rarely, but sometimes the databasescheme or physical organization must be modified.

Granting of authorization fordata access: granting different

types of authorization for data access to various users Integrity constraint specification: generating integrityconstraints. These are consulted by the database managermodule whenever updates occur.

In case of small organization the role ofDBA isperformed by a single person and in case of largeorganizations there is a group ofDBA's who shareresponsibilities.



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Database Designers

They are responsible for identifying the data tobe stored in the database and for choosingappropriate structure to represent and store the

data. It is the responsibility of database designers to

communicate with all prospective of thedatabase users in order to understand their

requirements so that they can create a designthat meets their requirements.


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Application Programmers

Application Programmers are responsible forwriting application programs that use the database.

These programs could be written in GeneralPurpose Programming languages such asVisual Basic, Developer, C, FORTRAN, COBOLetc. to manipulate the database.

These application programs operate on the datato perform various operations such as retaininginformation, creating new information, deletingor changing existing information.


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End Users

End Users are the people who interact with thedatabase through applications or utilities. Thevarious categories of end users are:

Casual EndUsers - These Users occasionallyaccess the database but may need differentinformation each time. They use sophisticateddatabase Query language to specify theirrequests.

For example: High level Managers who accessthe data weekly or biweekly.


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End Users Native EndUsers - These users frequently query and

update the database using standard types ofQueries.The operations that can be performed by this class ofusers are very limited and effect precise portion of thedatabase.

For example: - Reservation clerks for airlines/hotelscheck availability for given request and makereservations. Also, persons using Automated TellerMachines (ATM's) fall under this category as he hasaccess to limited portion of the database.

Standalone endUsers/On-line EndUsers - Thoseend Users who interact with the database directly via on-line terminal or indirectly through Menu or graphicsbased Interfaces.

For example: - User of a text package, librarymanagement software that store variety of library data

such as issue and return of books for fine purposes.


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Data Abstraction

The major purpose of a database systemis to provide users with an abstractview of the system.

The system hides certain details of howdata is stored and created andmaintained

Complexity should be hidden fromdatabase users.


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Data Abstraction

There are several levels of abstraction: Physical Level:

How the data are stored. E.g. index, B-tree, hashing. Lowest level of abstraction. Complex low-level structures described in detail.

Conceptual Level: Next highest level of abstraction. Describes whatdata are stored. Describes the relationships among data. Database administrator level.

View Level:

Highest level. Describespartof the database for a particular group of users. Can be many different views of a database. E.g. tellers in a bank get a view of customer accounts, but not of payroll

data.


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Data Abstraction


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Data Independence

The ability to modify a scheme definition in onelevel without affecting a scheme definition in a

higher level is called data independence.

There are two kinds:

Physical data independence The ability to modify the physical scheme without causing

application programs to be rewritten

Modifications at this level are usually to improve performance

Logical data independence The ability to modify the conceptual scheme without causing

application programs to be rewritten

Usually done when logical structure of database is altered


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Data Models

Data models are a collection ofconceptual tools for describing data, datarelationships, and data semantics anddata constraints. There are three differentgroups:

Object-based Logical Models.

Record-based Logical Models. Physical Data Models.


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Record-based Logical Models Record-based logical models:

describe data at the conceptual and viewlevels.

are used to Specify overall logical structure of the

database, and Provide a higher-level description of the

implementation.

Named so because the database isstructured in fixed-format records of severaltypes.

Each record type defines a fixed number offields, or attributes.


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Record-based Logical Models Each field is usually of a fixed length (this

simplifies the implementation). Record-based models do not include a

mechanism for direct representation of codein the database.

Separate languages associated with themodel are used to express database queriesand updates.

The three most widely-accepted models arethe relational, network, and hierarchical.


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The Relational Model

Data and relationships are representedby a collection oftables.

Each table has a number of columnswith unique names, e.g. customer,account.


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The Network Model

Data are represented by collections ofrecords.

Relationships among data are representedby links.

Organization is that of an arbitrary graph.


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The Hierarchical Model

Similar to the network model.

Organization of the records is as acollection oftrees, rather than arbitrarygraphs.


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The relational model does not use pointersor links, but relates records by the valuesthey contain. This allows a formalmathematical foundation to be defined


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Concepts of Database

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