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Topics:

1. DESCRIBING AND STORING DATAIN A DATABASE

2. Queries in DBMS

3. Structure of DBMS

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Database Schema (intension)

description of the database is specified during

database designDatabase State (extension of the schema)

current state of the database

actual data instances (occurrences) in a DB

changes over time by updateInitially, a database is empty state with no data

then, populate (load) the database with data

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Schemas, Instances and Database State

Database Schema (meta-data): The description of a database.

Includes descriptions of the database structure and the constraintsthat should hold on the database.

Schema Diagram: A diagrammatic display of (some aspects of ) a

database schema.

Database Instance: The actual data stored in a database at a

particular moment in time. Also called database state ( or

occurrence, snapshot)

The database schema changes very infrequently. The database

state changes every time the database is updated.

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Schema diagram for UNIVERSITY database

schema construct

Known data:

name of record types, data items

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Figure 1.2

UNIVERSITY Database

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Data model

Collection of high level data description without low level storagedetails eg Relational data model

Semantic data model describes a real application scenario.

Eg: Entity relationship model

Relational model

Relations called as records

Data are described as Schema

Schema specifies its name, name of the field and type of each field.

Eg: Students (Sid:String,name:String,login:String,age:integer)

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An example of Students relation is shown below

Sid Name Login age

53666 Jones jones@cs 18

53688 Smith smith@ee 18

53650 Smith smith@math 19

53831 Madayan madayan@music 11

53832 Mary mary@music 12

Integrity constraint: Records in the relation should satisfy thiscondition

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Other Data Models

Hierarchical model

IMS DBMS of IBM Trees of records: one-to-many relationships

Limitations:

Requires duplicating records (e.g. many-to-many

relationship)Network modelIDS & IDMS

similar to the hierarchical database with the implementation

of many-to-many relationships

Relational Model- DB2 , Informix,Oracle,Sybase, MS Access

Object-Oriented ModelObjectstore & Versant

Objects (collection of data items and procedures) and interactions between

them.

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External

schema 1

External

schema 2

External

schema 3

Conceptual schema

Physical schema

Disk

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Describes the stored data in terms of the data model of the DBMS.This leads to conceptual database design.

Also called as logical schema

Represents data as a set tables.

DBMS maps from conceptual schema to physical schema

Example:

Student(RegNo:Integer, Name:String, Sem:Integer, Branch:String)

Faculty(Fid:Integer, FName:String, Salary:Float)

Course(CourseNo:Integer, CName:String, Credit:Integer,Dept:String)

Section(SecId:Integer, CourseNo:Integer, Sem:Integer, Year:Integer,Instructor:String)

GradeReport(RegNo:Integer, SecId:Integer, Grade:Char)

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Describes the actual storage details of the relations described in

conceptual schema.

Creates Indexes on First Column of Students, Faculty and Course

relations.

This leads to the physical database design.

Physical schema specifies additional storage devices

Describes how the relations described in conceptual schema are

stored on secondary storage devices such as disks and tapes.

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Describes several views of the database based on the databasemodel.

Several external schemas are possible for a single database.

Each view is based upon the user requirements.

Data access to be customized at a level of individual users or groups

of users

Each conceptual schema consist of collection of one or more views.

Example:

StdGrade(RegNo:Integer, Name:String,Sem:Integer, Grade:Char)

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A Database has

One conceptual schema

One Physical schema

Many External Schema each tailored to aparticular group of users.

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The data independence is the ability to change theschema at one level of a database system with outchanging the schema at a higher level.

Logical data Independence

Capacity to change the conceptual schema withouthaving to change the external schema, is called asthe logical data independence.

With out changing the application programs, one canchange the logical schema.

http://www.doag.de/orafaq/glossary/faqgloss.htmhttp://www.doag.de/orafaq/glossary/faqgloss.htmhttp://www.doag.de/orafaq/glossary/faqgloss.htm

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Example

Suppose the Faculty relation is modified as:Faculty_Public(Fid:Integer, FName:String,Office:Integer)

Faculty_Private(Fid:Integer, Salary:Float)

Any view designed before this modification canstill retrieve the data with little modification(relation name) and obtain the same answer.

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Physical Data Independence

There are occasions for changing the internalstructures for improved performance of theretrieval of data.

Any change introduced to the internal schemaor physical schema will not affect the otherschemas.

Change the internal schema without having tochange the conceptual schema

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Database Languages DDL Data Definition Language

SDL Storage Definition Language

VDL View Definition Language

DML Data Manipulation Language

(For data manipulations like insertion,deletion, update, retrieval etc.)

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Database Languages and Interfaces

Data Definition Language (DDL)Used by the DBA and database

designers to specify the conceptual schema of a database.

In many DBMSs, the DDL is also used to define internal and external

schemas (views).

In some DBMSs, separate storage definition language (SDL) andview definition language (VDL) are used to define internal and

external schemas.

Data Manipulation Language (DML)Used to specify database

retrievals and updates (insertion, deletion, modifications)

- Alternatively, stand-alone DML commands can be applied directly

(query language).

provide appropriate languages and interfaces for each category of users.

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Data Manipulation Language (DML)

Data Manipulation Language (DML) is a family ofcomputer languages used by computer programs and/ordatabase users to insert, delete and update data in adatabase.

Read-only querying, i.e. SELECT, of this data is a part ofDML

SELECT ... INTO INSERT UPDATE DELETE

http://en.wikipedia.org/wiki/Computer_languagehttp://en.wikipedia.org/wiki/Computer_languagehttp://en.wikipedia.org/wiki/Databasehttp://en.wikipedia.org/wiki/Databasehttp://en.wikipedia.org/wiki/Select_(SQL)http://en.wikipedia.org/wiki/Insert_(SQL)http://en.wikipedia.org/wiki/Update_(SQL)http://en.wikipedia.org/wiki/Delete_(SQL)http://en.wikipedia.org/wiki/Delete_(SQL)http://en.wikipedia.org/wiki/Update_(SQL)http://en.wikipedia.org/wiki/Insert_(SQL)http://en.wikipedia.org/wiki/Select_(SQL)http://en.wikipedia.org/wiki/Databasehttp://en.wikipedia.org/wiki/Computer_language

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Queries in DBMS

Consider a sample University DatabaseSome questions user might ask:

1. What is the name & address of student with ENO =06701442010 ?

2. How many students are enrolled in BBA 108?

3. How many students have more than 70% ?

Such questions involving the data stored in a DBMS arecalled Queries.

DBMS provides specialized language called QueryLanguage

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Relational Calculus is a formal query language basedon mathematical logic.

Relational Algebra is another formal query languagebased on collection of Operators .

DBMS takes great care to evaluate queries as efficiently

as possible.Indexes can be used to speed up Queries.

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Relational Calculus Relational calculus consists of two calculi, the

tuple relational calculus and the domainrelational calculus, that are part of therelational model for databases

provides a declarative way to specify databasequeries.

This in contrast to the relational algebra which isalso part of the relational model but provides amore procedural way for specifying queries.

http://en.wikipedia.org/wiki/Tuple_relational_calculushttp://en.wikipedia.org/wiki/Domain_relational_calculushttp://en.wikipedia.org/wiki/Domain_relational_calculushttp://en.wikipedia.org/wiki/Relational_modelhttp://en.wikipedia.org/wiki/Relational_algebrahttp://en.wikipedia.org/wiki/Relational_algebrahttp://en.wikipedia.org/wiki/Relational_modelhttp://en.wikipedia.org/wiki/Domain_relational_calculushttp://en.wikipedia.org/wiki/Domain_relational_calculushttp://en.wikipedia.org/wiki/Tuple_relational_calculus

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Relational Algebra Relational algebras received little attention until

the publication ofE.F. Codd's relational modelof data in 1970. Codd proposed such analgebra as a basis for database querylanguages

The six primitive operators of Codd's algebra

are the selection, theprojection, the Cartesianproduct(also called the cross productor crossjoin), the set union, the set difference, and therename

http://en.wikipedia.org/wiki/Edgar_F._Coddhttp://en.wikipedia.org/wiki/Relational_modelhttp://en.wikipedia.org/wiki/Relational_modelhttp://en.wikipedia.org/wiki/Selection_(relational_algebra)http://en.wikipedia.org/wiki/Projection_(relational_algebra)http://en.wikipedia.org/wiki/Cartesian_producthttp://en.wikipedia.org/wiki/Cartesian_producthttp://en.wikipedia.org/wiki/Set_unionhttp://en.wikipedia.org/wiki/Set_differencehttp://en.wikipedia.org/wiki/Rename_(relational_algebra)http://en.wikipedia.org/wiki/Rename_(relational_algebra)http://en.wikipedia.org/wiki/Set_differencehttp://en.wikipedia.org/wiki/Set_unionhttp://en.wikipedia.org/wiki/Cartesian_producthttp://en.wikipedia.org/wiki/Cartesian_producthttp://en.wikipedia.org/wiki/Projection_(relational_algebra)http://en.wikipedia.org/wiki/Selection_(relational_algebra)http://en.wikipedia.org/wiki/Relational_modelhttp://en.wikipedia.org/wiki/Relational_modelhttp://en.wikipedia.org/wiki/Edgar_F._Codd

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Queries

Find all courses that Mary takes

What happens behind the scene ? Query processor figures out how to answer the

query efficiently.

SELECT C.nameFROM Students S, Takes T, Courses CWHERE S.name=Mary and

S.ssn = T.ssn and T.cid = C.cid

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Structure of a DBMS

A typical DBMS has a layeredarchitecture.

This is one of several possible

architectures; each system has its ownvariations.

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Webforms Application front

ends

SQL interface

Plan executor

Operator Evaluator Optimizer

parserShows interaction

Query EvaluationEngine

TransactionManager

Lock manager

Files and Access

Buffer Manager

Disk space

Manager

Recovery

Manager

Index Files

Data FilesSystem catalog

Shows references

Database

Architecture of DBMS

SQL commands

Concurrency Control

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DBMS accepts SQL commands generated from variety of user

interfaces

Query is parsed and then presented to Query optimizer

An execution plan is produced which is a blueprint for evaluatingquery

The code that implements query lies above file and access method

layer in which heap files are managed

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Below it is buffer manager which handles read requests of files

from disk to memoryThe lowest layer is Disk Space Manager

DBMS components associated with concurrency control and crash

recovery includes:

Transaction Manager which ensures transaction request and

schedules

Lock Manager which handles locks grant and release on data

items

Recovery Manager which maintains log and restores system to

consistent state after system crash.