relational algebra,calculas

8/14/2019 relational algebra,calculas

1/29

Silberschatz, Korth and Sudarshan1.1Database System Concepts

Chapter 1: Introduction

Purpose of Database Systems

View of Data

Data Models

Data Definition Language

Data Manipulation Language

Transaction Management

Storage Management

Database Administrator

Database Users

Overall System Structure


2/29


Database Management System (DBMS)

DATABASE:Collection of interrelated data.

DATA: known facts that can be recorded and that have implicit meaning.

DBMS:Set of programs to access the data

DBMS contains information about a particular enterprise

DBMS provides an environment that is both convenientand efficientto use.

Database Applications: Banking: all transactions

Airlines: reservations, schedules

Universities: registration, grades

Sales: customers, products, purchases

Manufacturing: production, inventory, orders, supply chain

Human resources: employee records, salaries, tax deductions

Databases touch all aspects of our lives


3/29


Functions of DBMS

Defining a database:

Specifying the data structures and constraints for the data to be stored inthe database.

Constructing the database:

Storing data on some storage medium

Manipulating a database:Querying (retrieving, updating)

Sharing a database :

Multiuser access

Protecting a database: System protection -> against hardware or software malfunction

Security protection -> against unauthorized access


4/29


Characteristics of Database approach

Self-describing nature of a database system:

Database system consists of complete definition or description (structure of each file, the

type and storage format of each data item and various constraints on data) of thedatabase structure or constraints.

This definition is stored in DBMS catalog

The information stored in catalog is calledmeta-data.

Insulation between program and data, and data abstraction:

In traditional file processing, the structure of data files is embedded in applicationprograms, so any changes to structure of a file may require changing all programs thataccess this file.

Support of Multiple Views of the Data:

A database typically has many users, each of whom may require a different perspective

or view of the database. Sharing of Data and Multiuser Transaction Processing:


5/29


Purpose of Database System

In the early days, database applications were built on top offile systems

Drawbacks of using file systems to store data:

Data redundancy and inconsistency

Multiple file formats(programs may be written in different

programming languages), duplication of information in different files. Change in one file doesntmake changes in other file.

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 partof program code

Hard to add new constraints or change existing ones


6/29


Purpose of Database Systems (Cont.)

Drawbacks of using file systems (cont.)

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

Database systems offer solutions to all the above problems


7/29Silberschatz, Korth and Sudarshan1.7Database System Concepts

Levels of Abstraction/3-Schema Architecture

Physical level or internal level

describes howa record (e.g., customer) is stored.

Lowest level of abstraction.

Describes the complete details of data storage and access paths for the database.

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

describes the structure of the whole database for a community of users.

hides the details of physical storage structures and concentrates on describing entities,data types, relationships, user operations, and constraints.

View level:

Views can also hide information (e.g., salary) for security purposes.

describes the part of the database that a particular user group is interested in and hidesthe rest of the database from that user group



View of Data

An architecture for a database system



Instances and Schemas

Similar to types and variables in programming languages

Schemathe overall design of database

e.g., the database consists of information about a set of customers andaccounts and the relationship between them)

Physical schema: describes database design at the physical level

Logical schema: describes database design at the logical level

Instance the collection of information stored in the database at aparticular point in time



Data Independence

capacity to change the schema at one level of a database system withouthaving to change the schema at the next higher level.

Logical data independence

is the capacity to change the conceptual schema without having to change externalschemas or application programs.

We may change the conceptual schema to expand the database (by adding a recordtype or data item), or to reduce the database (by removing a record type or data item).

Physical data independence

is the capacity to change the internal schema without having to change the conceptual(or external) schemas.

Changes to the internal schema may be needed because some physical files had to bereorganizedfor example, by creating additional access structuresto improve theperformance of retrieval or update.



Database Users

Actors on the scene:

Database Administrators

Database Designers

End Users

System Analysts and Application Programmers (Software Engineers)

Actors behind the scene: DBMS system designers and implementers

Tool developers

Operators and maintenance personnel



DATABASE ADMINISTRATOR

Coordinates all the activities of the database system; thedatabase administrator has a good understanding of theenterprises information resources and needs.

Database administrator's duties include:

Schema definition Storage structure and access method definition

Schema modification

Granting user authority to access the database

Routine maintenance (periodic backup, upgrading disk space,

monitoring running jobs on database)

Specifying integrity constraints

Acting as liaison with users

Monitoring performance and responding to changes in requirements



DATABASE DESIGNERS:

responsible for identifying the data to be stored in the database and forchoosing appropriate structures to represent and store this data.

These tasks are mostly undertaken before the database is actuallyimplemented and populated with data.

It is the responsibility of database designers to communicate with all

prospective database users, in order to understand their requirements, and tocome up with a design that meets these requirements.

In many cases, the designers are on the staff of the DBA and may beassigned other staff responsibilities after the database design is completed.

Database designers typically interact with each potential group of users and

develop a view of the database that meets the data and processingrequirements of this group.



End users are the people whose jobs require access to the database forquerying, updating, and generating reports; the database primarily existsfor their use. There are several categories of end users:

Casual end users occasionally access the database, but they may need different information each time.

They use a sophisticated database query language to specify their requests.

Naive or parametric end users

main job function revolves around constantly querying and updating the database, using

standard types of queries and updatescalled canned transact ionsthat have beencarefully programmed and tested. The tasks that such users perform are varied:

Bank tellers check account balances and post withdrawals and deposits.

Sophisticated end users

include engineers, scientists, business analysts, and others who thoroughly familiarize

themselves with the facilities of the DBMS so as to implement their applications to meettheir complex requirements.

Stand-alone users

maintain personal databases by using ready-made program packages that provide easy-to-use menu- or graphics-based interfaces.

An example is the user of a tax package that stores a variety of personal financial datafor tax purposes.



System Analysts and Application Programmers (Software Engineers)

System analysts determine the requirements of end users, especially naive andparametric end users, and develop specifications for canned transactions that meet theserequirements.

Appl ica t ion programmers implement these specifications as programs; then they test,debug, document, and maintain these canned transactions.



Actors behind the Scene

DBMS system designers and implementers

persons who design and implement the DBMS modules and interfacesas a software package.

modules for implementing the catalog, query language, interfaceprocessors, data access, concurrency control, recovery, and security.



Tool developers

include persons who design and implement toolsthe software packagesthat facilitate database system design and use, and help improveperformance.

Tools are optional packages that are often purchased separately.

They include packages for database design, performance monitoring, naturallanguage or graphical interfaces, prototyping, simulation, and test datageneration.



Operators and maintenance personnel

are the system administration personnel who are responsible for theactual running and maintenance of the hardware and softwareenvironment for the database system.



Data Models

A collection of conceptual tools for describing

data

data relationships

data semantics

data constraints

Entity-Relationship model

Relational model : uses a collection of table to represent both data andrelationships.

Other models:

object-oriented model (extension of ER, with notion of encapsulation, methods,objects)

semi-structured data models (where individual data item of same type havedifferent set of attributes.) Older models: network model and hierarchical model

where the hierarchical database model structures data as a tree of records, with each record having oneparent record and many children, the network model allows each record to have multiple parent andchild records, forming a generalized graph structure.



Entity-Relationship Model

Example of schema in the entity-relationship model



Entity Relationship Model (Cont.)

E-R model of real world

Entities (objects)

E.g. customers, accounts, bank branch

Relationships between entities

E.g. Account A-101 is held by customer Johnson

Relationship set depositorassociates customers with accounts Widely used for database design

Database design in E-R model usually converted to design in therelational model (coming up next) which is used for storage andprocessing



Relational Model

Example of tabular data in the relational model

customer-

nameCustomer-

id

customer-

street

customer-

city

account-

number

Johnson

Smith

Johnson

Jones

Smith

192-83-7465

019-28-3746

192-83-7465

321-12-3123

019-28-3746

Alma

North

Alma

Main

North

Palo Alto

Rye

Palo Alto

Harrison

Rye

A-101

A-215

A-201

A-217

A-201

Attributes


23/29


A Sample Relational Database


24/29


Data Definition Language (DDL)

Specification notation for defining the database schema

E.g.create tableaccount(

account-number char(10),balance integer)

DDL compiler generates a set of tables stored in a data

dictionary

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

database schema

Data storage and definitionlanguage

language in which the storage structure and access methodsused by the database system are specified

Usually an extension of the data definition language

Creaye, alter, drop, truncate, grant, revoke


25/29


Data Manipulation Language (DML)

Language for accessing and manipulating the data organized by

the appropriate data model DML also known as query language

Two classes of languages

Proceduraluser specifies what data is required and how to getthose data

Nonproceduraluser specifies what data is required withoutspecifying how to get those data

SQL is the most widely used query language

Insert, update, delete


26/29


SQL

SQL: widely used language

E.g. find the name of the customer with customer-id 192-83-7465select customer.customer-namefrom customerwhere customer.customer-id= 192-83-7465

E.g. find the balances of all accounts held by the customer with

customer-id 192-83-7465select account.balancefrom depositor, accountwhere depositor.customer-id= 192-83-7465 and

depositor.account-number = account.account-number

Application programs generally access databases through one of

Language extensions to allow embedded SQL

Application program interface (e.g. ODBC/JDBC) which allow SQLqueries to be sent to a database


27/29


Transaction Management

A transactionis a collection of operations that performs a single

logical function in a database application Transaction-management component ensures that the database

remains in a consistent (correct) state despite system failures(e.g., power failures and operating system crashes) andtransaction failures.

Concurrency-control manager controls the interaction among theconcurrent transactions, to ensure the consistency of thedatabase.


28/29


Storage Management

Storage manager is a program module that provides the

interface between the low-level data stored in the database andthe application programs and queries submitted to the system.

The storage manager is responsible to the following tasks:

interaction with the file manager

efficient storing, retrieving and updating of data


29/29

Overall System Structure

relational algebra,calculas

Documents