chapter1-introduction to dbms

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CCourseourse 11

IIntroduction tontroduction to DDBMSBMS

Dr. Mohsen Rouached

CComputeromputer SScience (CScience (CS 420420))

DDatabaseatabase MManagement anagement SSystemsystems (DBMS)(DBMS)



Table of contents Definition of DBMS Objective of DBMS

Database System Applications Purpose of Databases

Limitation of the file system based databases

View of Data in Databases

Data Models Database Languages Database Users and Administrator



Definition of DBMS A database management system (DBMS) is

a collection of interrelated data (database)and

a set of programs to access those data.



Objective of DBMS Provide a way for efficient data management

storage and retrieval of the information

Manage large bodies of information Defining structures for the storage of information

Providing the mechanisms for the manipulation of theinformation

Ensure the safety of the information stored Avoid abnormal results when sharing the DB

among several users



Database System Applications Banking: all transactions

Airlines: reservations, schedules

Universities: registration, grades, student information Sales: customers, products, purchases

Manufacturing: production, inventory, orders, supplychain

Human resources: employee records, salaries, taxdeductions

Telecommunications: keeping record of calls made,generating monthly bills



Purpose of Database Systems

Limitations of File Systems (1/3)

In the early days, database applications were built on topof file systems Data is stored in separate files Data is manipulated by a set of application programs

As time goes by, the number of files and applicationprograms increases increasing complexity of database application management





Drawbacks of using file systems to store data Data redundancy and inconsistency

Redundancy: duplication of information in different files Inconsistency: the various copies of the same data may be different

Difficulty in accessing data Need to write a new program to carry out each new task

Data isolation multiple files and formats Because data are scattered in various files, and files may be in different

formats, writing the appropriate information to retrieve the appropriate data isdifficult

Integrity problems Data values stored in the database must satisfy certain types of consistency

constraints Integrity constraints (e.g. account balance > 0) become part of program code Those constraints are forced by the database system developer Hard to add new constraints or change existing ones





Drawbacks of using file systems to store data Atomicity of updates

Failures may leave database in an inconsistent state with partialupdates carried out

E.g. transfer of funds from one account to another should eithercomplete or not happen at all

Concurrent access by multiple users Concurrent accessed needed for performance Uncontrolled concurrent accesses can lead to inconsistencies

E.g. two people reading a balance and updating it at the same time Security problems

Not every user of the database should be able to access all the data

Database systems offer solutions to all previous problems



View of Data in a Database

Recall that

a database management system (DBMS) is a collectionof interrelated data and a set of programs to access andmodify those data.

A major purpose of a database system is to provide userswith an abstract view of data

The system hides certain details of how the data arestored and maintained




The Need for Data Abstraction

The objective of data abstraction is to

simplify the interaction with the databasefrom the user perspective

Most of database users are not computertrained

However, more complexity from the designperspective

Complex data structure to represent data




Data Abstraction Levels

Physical Level: describes how the data is

actually stored Describes low level data structures

Logical Level: describes what data arestored in the database and what

relationships exist between data View Level: describes only part of the

entire database




Data Abstraction Levels

Physical Level: describes how the data is actually stored Describes low level data

structures Logical Level: describes what

data are stored in thedatabase and whatrelationships exist betweendata

View Level: describes onlypart of the entire databaseH

ow to store

What to store




Analogy with data types in programming

Consider high level programming

Physical Level: block of consecutivestorage locations (e.g. bytes) The compiler hides this level of details

from programmers

Logical Level: each record is describedby a type definition Programmers work at this level of

abstraction

Similar to database administrator

View Level: computer users only see aset of applications without details ondata types Similar to users of database

e.g. some database users are notallowed to see the salary of workers for security reasons.

type customer = recordname : string;street : string;

city : integer;end;



Data Models

Definition

The Data Model: a collection of conceptual

tools for describing Data

Data relationship

Data semantics

Consistency constraints A data model provide a way to describe the

design of a database at the physical, logical

and view level



Data Models

Definition

Data models can be classified in four

categories1. Relational model

2. The Entity-Relation model

3. Object-based Data model

4. Semi Structured Data model

Other models: network model,hierarchical model



Data Models

The Relational Model

The Relational Model: uses a collection of tables

to represent both data and the relationship amongthose data.

Each table has multiple column and each column has a

unique name (Attribute)

The most widely used as a data model for databases Vast majority of current database systems are based on

the relational model



Data Models

The Relational ModelSample of a Relational databaseSample of a Relational database

Attributes



Data Models

The Entity-Relationship Model

The Entity-Relationship (E-R) Model: it is based on the

perception of a real world that consists of a collection

basic objects called entities and of relationships among

those objects.

An entity is a ³thing´ or ³object´ from the real world that is

distinguishable from other objects

e.g. c ustomers, acc ounts, bank branc h

A relationship is an association among several entities

e.g. the acc ount Ac1234 is held by the c ustomer Mr. Abdullah

This model is also widely used in database design



Data Models

The Entity-Relationship Model

The Entity-Relationship (E-R) Model: it is basedon the perception of a real world that consists of acollection basic objects called entities and of relationships among those

Rectangles: represent the set of entities

Ellipses: represent the attributes of anentity Diamonds: represent the set of relationships among a member from eachof several entity sets Lines: link the attributes to entity setsand entity sets to relationships

entity

Relationship

entity

Attributes



Data Language

Definition

A database system provides a data definition language to specify the

database schema a data manipulation language to express

database queries and update

In practice, both languages are not separate e.g. SQL (Structured Query Language)



Data Definition Language (DDL) Data Definition Language: specifies the notation for defining the

database schema

e.g.

create table acc ount (acc ount-number char (10),balanc e integer

)

DDL compiler generates a set of tables stored in a d ata d ictionary

Data dictionary contains metadata (i.e., data about data)

The schema of a table is an example of metadata Data storage and definition language

Usually an extension of the data definition language

Language in which the storage structure and access methods used by thedatabase system are specified

Hidden from the user of the database



Data Manipulation Language (DML)

Language for accessing and manipulating the

data organized by the appropriate data model

DML also known as query language

Data manipulation is:

The retrieval of information stored in the database

The insertion of new information into the database

The deletion of the information from the database

The modification (update) of the information from the

database



Data Manipulation Language (DML) Two classes of languages

Procedural ± user specifies what data is required and how to get

those data

Declarative (Nonprocedural) ± user specifies what data is

required without specifying how to get those data (e.g. SQL)

Easy to learn and to use

The database system has to figure out an efficient means of

accessing the data

SQL is the most widely used query language

A Query is a statement requesting the retrieval of the

information



SQL: Some Examples SQL: widely used non-procedural language

e.g. find the name of the customer with customer-id 192-83-7465select custom er.custom er-nam e

from custom erwhere custom er.custom er-id = 192-83-7465

e.g. find the balances of all accounts held by the customer withcustomer-id 192-83-7465

select a ccount .bala nc efrom depositor , a ccount

where d epositor .custom er-id = 192- 83- 7465 andd epositor .a ccount-num ber = a ccount .a ccount-num ber

Application programs generally access databases through one of

Language extensions to allow embedded SQL

Application Program Interface (API) (e.g. ODBC/JDBC) which allow SQL

queries to be sent to a database



Database Users People who work with a database can be

categorized as

database users or database administrators

Users are differentiated by the way they expect to interact with the system



Database Users Four types of database users

Naïve users: invoke one of the permanent application programs that have beenwritten previously

E.g. Bank cashier, people accessing a database over the web

The typical user interface for naïve user is a form interface where the user just fill in theappropriate fields of the form

Application programmers: computer professional that write applicationprograms

Choose a tool from many tools to build a user interface to the database

Interact with the system through DML calls

Sophisticated users: Interact with the system without writing programs Form their requests using a database query language (e.g. Analysts)

Specialized users: write specialized database applications that do not fit intothe traditional data processing framework

E.g. computer-aided design systems, expert systems, environement modeling systems



Database Administrators The Database Administrator (DBA) Coordinates all the activities of the

database system

The DBA has a good understanding of the enterprises information

resources and needs The DBA has central control over the system

Database administrator's duties include:

Schema definition, specifying integrity constraints

Storage structure and access method definition

Schema and physical organization modification

Granting user authority to access the database

Acting as liaison with users

Monitoring performance and responding to changes in requirements

Routine maintenance (DB backup, Ensuring free space for the DB, etc )



Transaction Management A transaction is a collection of operations that performs s single logical

function in a database application E.g. Money transfer from an account to another

Each transaction must satisfy the following properties: Atomicity: all -or-non e, which mea n s tha t the t r a n sa ction successfull y

co mpl etes if and only if all the t a sks o f the t r a n sa ction s a r e successfull yco mpl eted

E.g. in case of a money transfer, the transaction is valid if both debit and credit operations occur.

Consistency: the final state of the system must be correct and compatiblewith the previous status before the transaction is made.

E.g. after a credit transaction, the balance must be updated accordingly to theoperation

IIsolationsolation. It ensures that concurrently executing transactions are isolatedfrom one another, so that each transaction has the impression that no other transaction is executing concurrently with it.

That is, for every pair of transactions T i and T j , it appears to T i that either T j , finished execution before T i started, or T j started execution after T i finished.

Durability: the value in the database must persist, even in case of a system failure.



Database System Structure A database system has several sub-systems

Transaction Manager Responsible for ensuring that the database remains in a

consistent (correct) state despite systems failure.

Ensures that concurrent transaction execution proceed without conflicting

Query Processor Compile and executes DDL and DML statements

Storage Manager

Provides the interface between the low-level data stored in thedatabase and the application programs and queries submittedto the system



Application Architectures (Most of) Database systems are accessed

remotely through a network

The Client/Server Model Client machines, on which remote database users work

Server machines, on which the database system runs

DatabaseClient machines

Server machines

Network



Application Architectures Two-Tiered Architecture

The application is partitioned intoa component that resides at theclient machine

It remotely invokes the databaseserver machine through querylanguages statements

E.g. using ODBC, JDBC for interaction between the client andthe server

Three-Tiered Architecture The client machine acts as a

front-end and does not containany direct database calls

The client machinecommunication with the server application through an interfaceof forms (not direct queries)

The server applicationcommunicate with a database

system to access data

application

User

Database systems

NetworkNetwork

Two-Tiered Architecture

Application client

User

Database systems

NetworkNetwork

Three-Tiered Architecture

Application server



Instances and Schemas Instance of a database

The collection of information stored in the database at a

particular moment is called an instance. Schema of database

The overall design of the database is called schema.

Physical Schema

Describes the data design at the physical level

Logical Schema

Describes the data design at the logical level



Review Terms Database management

system (DBMS)

File Systems Data inconsistency

Consistency constraints

Data views/ data

abstraction Database instance

Data models ((E-R), Relational)

Database languages (DDL, DML)

Data Dictionary

Metadata

Application program interface

Database administrator

Transactions

Concurrency Client and servers machines



Exercises Q.1 List four significant differences

between files systems and a DBMS



Exercises Q.2 What are the steps that you would take in setting up a

database for a particular enterprise? R2.

Collecting the required information that must be stored in thedatabase

Define the structure of the schema of a database, which definesthe logical and physical structure of the data

Define the method for manipulating the database Establish the relation between data Granting authorization for data access for the case of multi-user

database systems Creating different views of the database for different users of the

database



Exercises Q.3 What is the purpose of having a

DBMS?

R3. the primary goal of a DBMS is toprovide an environment that is both

convenient and efficient for people touse in retrieving and storinginformation



Exercises Q.4 Define a transaction. Explain the

meaning of atomicity and illustrate it on example.

R4. A transaction is a collection of

operations that performs s single logicalfunction in a database application

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