Database Management System- Chapter 1

 

 

Chapter 1:  Introduction

 

n Purpose of Database Systems

n View of Data

n Data Models

n Data Definition Language

n Data Manipulation Language

n Transaction Management

n Storage Management

n Database Administrator

n Database Users

n Overall System Structure

 

Database Management System (DBMS)

 

n Collection of interrelated data

n Set of programs to access the data

n DBMS contains information about a particular enterprise

n DBMS provides an environment that is both convenient and efficient to use.

n Database Applications:

H Banking: all transactions

H Airlines: reservations, schedules

H Universities:  registration, grades

H Sales: customers, products, purchases

H Manufacturing: production, inventory, orders, supply chain

H Human resources:  employee records, salaries, tax deductions

n Databases touch all aspects of our lives

 

Purpose of Database System

 

n In the early days, database applications were built on top of file systems

n Drawbacks of using file systems to store data:

H Data redundancy and inconsistency

4 Multiple file formats, duplication of information in different files

           Difficulty in accessing data

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

H Data isolation — multiple files and formats

H Integrity problems

4 Integrity constraints  (e.g. account balance > 0) become part of program code

4 Hard to add new constraints or change existing ones

n Drawbacks of using file systems (cont.)

H Atomicity of updates

4 Failures may leave database in an inconsistent state with partial updates carried out

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

H Concurrent access by multiple users

4 Concurrent accessed needed for performance

4 Uncontrolled concurrent accesses can lead to inconsistencies

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

H Security problems

n Database systems offer solutions to all the above problems

 

 

Levels of Abstraction

 

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

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

            type customer = record
                                name : string;
                                street : string;
                                city : integer;
                    end;

n View level: application programs hide details of data types.  Views can also hide information (e.g., salary) for security purposes

n .

 

 

View of Data

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Instances and Schemas

 

n  Similar to types and variables in programming languages

n  Schema – the logical structure of the database

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

H Analogous to type information of a variable in a program

H Physical schema: database design at the physical level

H Logical schema: database design at the logical level

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

H Analogous to the value of a variable

n  Physical Data Independence – the ability to modify the physical schema without changing the logical schema

H Applications depend on the logical schema

H In general, the interfaces between the various levels and components should be well defined so that changes in some parts do not seriously influence others.

 

Data Models

 

n A collection of tools for describing

H data

H data relationships

H data semantics

H data constraints

n Entity-Relationship model

n Relational model

n Other models:

H object-oriented model

H semi-structured data models

H Older models: network model and hierarchical model

 

Entity-Relationship Model

 

Example of schema in the entity-relationship model

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

n E-R model of real world

H Entities (objects)

4 E.g. customers, accounts, bank branch

H Relationships between entities

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

4 Relationship set depositor associates customers with accounts

n Widely used for database design

H Database design in E-R model usually converted to design in the relational model (coming up next) which is used for storage and processing

 

 

Relational Model

 

n Example of tabular data in the relational model

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A Sample Relational Database

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Data Definition Language (DDL)

 

 

n Specification notation for defining the database schema

H E.g. 
  create table account (
             account-number    char(10),
             balance                 integer)

n DDL compiler generates a set of tables stored in a data dictionary

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

H  database schema

H Data storage and definition language

4  language in which the storage structure and access methods used by the database system are specified

4 Usually an extension of the data definition language

 

 

Data Manipulation Language (DML)

 

n Language for accessing and manipulating the data organized by the appropriate data model

H DML also known as query language

n Two classes of languages

H Procedural – user specifies what data is required and how to get those data

H Nonprocedural – user specifies what data is required without specifying how to get those data

n SQL is the most widely used query language

 

 

SQL

 

n SQL: widely used non-procedural language

H E.g. find the name of the customer with customer-id 192-83-7465
              select   customer.customer-name
              from     customer
              where  customer.customer-id = ‘192-83-7465’

H E.g. find the balances of all accounts held by the customer with customer-id 192-83-7465
              select   account.balance
              from     depositor, account
              where  depositor.customer-id = ‘192-83-7465’ and
                          depositor.account-number = account.account-number

n Application programs generally access databases through one of

H Language extensions to allow embedded SQL

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

 

 

 

Database Users

 

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

n Application programmers – interact with system through DML calls

n Sophisticated users – form requests in a database query language

n Specialized users – write specialized database applications that do not fit into the traditional data processing framework

n Naïve users – invoke one of the permanent application programs that have been written previously

H E.g. people accessing database over the web, bank tellers, clerical staff

 

Database Administrator

 

n Coordinates all the activities of the database system; the database administrator has a good understanding of the enterprise’s information resources and needs.

n Database administrator's duties include:

H Schema definition

H Storage structure and access method definition

H Schema and physical organization modification

H Granting user authority to access the database

H Specifying integrity constraints

H Acting as liaison with users

H Monitoring performance and responding to changes in requirements

 

Transaction Management

       

n A transaction is a collection of operations that performs a single logical function in a database application

n Transaction-management component ensures that the database remains in a consistent (correct) state despite system failures (e.g., power failures and operating system crashes) and transaction failures.

n Concurrency-control manager controls the interaction among the concurrent transactions, to ensure the consistency of the database.

 

Storage Management

n Storage manager is a program module that provides the interface between the low-level data stored in the database and the application programs and queries submitted to the system.

n The storage manager is responsible to the following tasks:

H interaction with the file manager

H efficient storing, retrieving and updating of data

 

 

 

 

 

 

Overall System Structure

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Application Architectures

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

§Two-tier architecture:  E.g. client programs using ODBC/JDBC to 
  communicate with a database

§Three-tier architecture: E.g. web-based applications, and
  applications built using “middleware”