unit 1 DBMS

8/3/2019 unit 1 DBMS

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Chapter 1: IntroductionChapter 1: Introduction


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Data baseData base

The Collection of data, usually referred to as the database, contains informationrelevant to an enterprise.

What is Data base Management System?

DBMS contains information about a particular enterprise

Collection of interrelated data

Set of programs to access the data An environment that is both convenientand efficientto use


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Database vs. File SystemsDatabase vs. File Systems


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WhatWhat areare thethe advantagesadvantages andand disadvantagesdisadvantages ofof DatabaseDatabase

ManagementManagement systemsystem overover conventionalconventional filefile system?system?

disadvantages of Database Management system over

conventional file system

Data redundancy and inconsistency

Multiple file formats, duplication of information in different files

Difficulty in accessing data

Need to write a new program to carry out each new task

Data isolation multiple files and formats

Integrity problems

Integrity constraints (e.g. account balance > 0) become buried in programcode rather than being stated explicitly

Hard to add new constraints or change existing ones


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disadvantagesdisadvantages ofof DatabaseDatabase ManagementManagement systemsystem over over

conventionalconventional filefile systemsystem

Atomicity of updates

Failures may leave database in an inconsistent state with partial updates

carried out

Example: Transfer of funds from one account to another should either

complete or not happen at all

Concurrent access by multiple users

Concurrent accessed needed for performance

Uncontrolled concurrent accesses can lead to inconsistencies

Example: Two people reading a balance and updating it at the same

time

Security problems Hard to provide user access to some, but not all, data


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advantagesadvantages ofofDatabaseDatabase ManagementManagement systemsystem overover conventionalconventional

filefile systemsystem

A DBMS scores over simple file systems in that it is quicker to access, easier to link

related data items, and much easier to maintain. It also makes more efficient use of

back-up memory resources.

In an office environment, some of the advantages would be improvement of

searching and the implementation of right management.

mainly, you should be aware that a database system, I suppose you're talking about a

relational one, is more per formant, therefore it's quicker to find a record which makesit more cheap than a file System,


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Levels of AbstractionLevels of Abstraction

Physical level: describes how a record (e.g., customer) is stored.

Logical level: describes data stored in database, and the relationships among the data.

type customer= record

customer_id:string;

customer_name :string;

customer_street:string;

customer_city:integer;

end;

View level: application programs hide details of data types. Views can also hide

information (such as an employees salary) for security purposes.


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View ofDataView ofData

Anarchitectureforadatabasesystem


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DefinesDBMSschemaatthreelevels:

- Internal schema attheinternallevel todescribedatastoragestructures andaccesspaths.Typicallyusesaphysicaldata model.

- Conceptual schema attheconceptuallevel todescribethestructure andconstraints forthe

whole database.Usesaconceptualoranimplementation datamodel.

- External schema attheexternallevel todescribethevarioususerviews.Usually uses the

samedatamodelastheconceptuallevel orhigh-leveldata model.


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Instances and SchemasInstances and Schemas

Schema the logical structure of the database

Example: The database consists of information about a set of customers and

accounts and the relationship between them)

Analogous to type information of a variable in a program

Conceptual organization of entire database as viewed by the database administrator

Physical schema: database design at the physical level Logical schema: database design at the logical level

Instance the actual content of the database at a particular point in time

Analogous to the value of a variable

Database files themselves are useless without the memory structures and processes

to interact with the database. Oracle defines the term instance as the memorystructure and the background processes used to access data from a database.


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Schema diagram forSchema diagram forUNIVERSITYUNIVERSITY databasedatabase

schemaconstruct


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UNIVERSITY DatabaseUNIVERSITY Database


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Sub SchemaSub Schema

That part of a database definition, to be viewed by particular applications, that

describes all or a subset of the data elements, record types, set types, and areas

defined in the schema. It is basically a portion of a schema - usually to show a

particular user department's portion of the database. It identifies a subset of areas,

sets, records, and data names defined in the database schema available to user

sessions.

Defines database portion "seen" by the application programs that actually producethe desired information from data contained within the database

The following are a few of the many reasons sub schemas are used:

Subschemasprovidedifferentviewsofthedatatotheuserandthe

programmer,whodonotneedtoknowallthedatacontainedintheentire

database.

Subschemasenhancesecurityfactorsandprohibitdatacompromise.


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Data Independence: It is the ability to modify a schema definition in one level withoutaffecting a schema definition in the next higher level. The interfaces between thevarious levels and components should be well defined so that changes in some partsdo not seriously influence others.

Allows data fields to be added, changed, and deleted from a database withoutnecessarily affecting existing application programs.

Two levels of data independence:

Physical data independence

Logical data independence

Physical data independence is the ability to modify the physical schema withoutmaking it necessary to rewrite application programs. E.G., changing from unblocked

to blocked record storage, or from sequential to random-access files. Logical data independence is the ability to modify the conceptual schema without

making it necessary to rewrite application programs. E.G., adding a new field to arecord. An application program's view hides this change from the program.

Data IndependenceData Independence


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UN

IVERS

ITYConceptual

SchemaSTUDENT(Name,Student Number,Class,Major)

COURSE(Course Name,Course Number,Credit,Dept)

PREREQUISITE(Course Number,Prerequisite Number)

SECTION (SectionId,Course Number,Semester,Year,Instructor)

GRADE_REPORT(Student Number,SectionId,Grade)

UNIVERSITYExternalSchema

TRANSCRIPT(Student Name,Course Number,Grade,Semester,Year,SectionId) derivedfrom

STUDENT,SECTION,GRADE_REPORT

PREREQUISITES(CourseName,Course Number,Prerequisites)

derivedfrom PREREQUISITE,COURSE


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Data ModelsData Models

Data Model: A set of concepts to describe the structure of a database, and

certain constraints that the database should obey.

A collection of tools for describing

Data

Data relationships

Data semantics

Data constraints

Relational model-Data model based on tables

The relational model uses a collection of tables to represent both data and the

relationships among those data.

Entity-Relationship data model (mainly for database design) The entity relationship (E-R) data model is based on a perception of a real

world that consists of a collection of basic objects, called entities and of

relationship among these objects.

Object-based data models (Object-oriented and Object-relational)-data model

based on the object-oriented programming paradigm

The Object-Oriented data model is another data model that has seen


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Data ModelsData Models

Semi structured data model (XML) The Semi structured data model permits the

specification of data where individual data items of the same type may have different

sets of attributes.

Other older models:

Network model-data model based on graphs with records as nodes and

relationships between records as edges

Hierarchical model-Data model based on trees


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Relational ModelRelational Model

Exampleoftabulardataintherelationalmodel

Attributes


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A Sample Relational DatabaseA Sample Relational Database


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The EntityThe Entity--Relationship ModelRelationship Model

Modelsanenterpriseasacollectionofentities andrelationships

Entity:athingorobjectintheenterprisethatisdistinguishablefromother

objects

Describedbyasetofattributes

Relationship:anassociationamongseveralentities

Representeddiagrammatically

byanentity-relations

hip diagram:


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ObjectObject--Relational Data ModelsRelational Data Models

Extend the relational data model by including object orientation and constructs to deal

with added data types.

Allow attributes of tuples to have complex types, including non-atomic values such as

nested relations.

Preserve relational foundations, in particular the declarative access to data, while

extending modeling power.

Provide upward compatibility with existing relational languages.


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Semi Structured Data ModelsSemi Structured Data Models

Semistructureddatamodelspermitthespecificationofdatawhereindividualdata

itemsofthesametypemayhavedifferentsetsofattributes.Thisisincontrastwith

thedatamodelsmentionedearlier. Whereeverydataitemofaparticulartypemust

havethesamesetofattributes.

XML: Extensible Markup Language

Definedbythe WWW Consortium(W3C)

Originallyintendedasadocumentmarkuplanguagenotadatabaselanguage

Theabilitytospecifynewtags,andtocreatenestedtagstructuresmadeXMLagreat

waytoexchangedata,notjustdocuments

XMLhasbecomethebasisforallnewgenerationdatainterchangeformats.

Awidevarietyoftoolsisavailableforparsing,browsingandqueryingXML

documents/data


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Data Manipulation Language (DML)Data Manipulation Language (DML)

Languageforaccessingandmanipulatingthedataorganizedbytheappropriatedata

model

DMLalsoknownasquerylanguage

TwotypesofDML

Procedural userspecifieswhatdataisrequiredandhowtogetthosedata

Alsocalledrecord-at-a-time(record-oriented) orlow-levelDML Mustbeembeddedinaprogramminglanguage.

Searchesforandretrievesindividualdatabaserecordsanduseslooping

andotherconstructsofthehostprogramminglanguagetoretrieve

multiple records.


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Declarative (nonprocedural) userspecifieswhatdataisrequiredwithout

specifyinghowtogetthosedata

Alsocalledset-at-a-time(set-oriented) orhigh-levelDML.

Canbeusedasastand-alonequerylanguageorcanbeembeddedina

programminglanguage.

Searchesforandretrievesinformationfrommultiplerelateddatabaserecordsin

asinglecommand.

Hostlanguage:general-purposelanguage

Datasublanguage:DML

C++

SQL

is

the

most

widely

used

query

language


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Data Definition Language (DDL)Data Definition Language (DDL)

Specificationnotationfordefiningthedatabaseschema

Example: create table account(

account-number char(10),

balance integer)

DDLcompilergeneratesasetoftablesstoredinadata dictionary

Datadictionarycontainsmetadata(i.e.,dataaboutdata)

Databaseschema

Datastorage and definition language

Specifiesthestoragestructureandaccessmethodsused

Integrityconstraints

Domainconstraints Referentialintegrity(references constraintinSQL)

Assertions Anassertionisanyconditionthatthedatabasemustalways

satisfy.Domainconstraintandreferentialintegrityconstraintarespecial

formsofassertions.

Authorization


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Database DesignDatabase Design

The process of designing the general structure of the database:

Logical Design Deciding on the database schema. Database design requires that we

find a good collection of relation schemas.

Business decision What attributes should we record in the database?

ComputerS

cience decision W

hat relation schemas should we have and howshould the attributes be distributed among the various relation schemas?

Physical Design Deciding on the physical layout of the database


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Query ProcessingQuery Processing

1. Parsingand

translation

2. Optimization

3. Evaluation


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Query Processing (Cont.)Query Processing (Cont.)

Alternative ways of evaluating a given query

Equivalent expressions

Different algorithms for each operation

Cost difference between a good and a bad way of evaluating a query can be enormous

Need to estimate the cost of operations

Depends critically on statistical information about relations which the database mustmaintain

Need to estimate statistics for intermediate results to compute cost of complex

expressions


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Database ArchitectureDatabase Architecture

The architecture of a database systems is greatly influenced by the underlyingcomputer system on which the database is running:

Centralized

Client-server

Parallel (multi-processor)

Distributed


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Overall System StructureOverall System Structure


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Two Tier and Three Tier ArchitectureTwo Tier and Three Tier Architecture


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Database AdministratorDatabase Administrator

Coordinates all the activities of the database system; the database

administrator has a good understanding of the enterprises information

resources and needs.

Database administrator's duties include:

Schema definition

Storage structure and access method definition

Schema and physical organization modification

Granting user authority to access the database

Specifying integrity constraints

Acting as liaison with users

Monitoring performance and responding to changes in requirements


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unit 1 DBMS