Database System Concepts Essay example

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Chapter 1: Introduction

Database System Concepts, 6th Ed.
©Silberschatz, Korth and Sudarshan
See www.db­book.com for conditions on re­use

Database Management System (DBMS)
 DBMS contains information about a particular enterprise


Collection of interrelated data



Set of programs to access the data



An environment that is both convenient and efficient to use

 Database Applications:


Banking: transactions



Airlines: reservations, schedules



Universities: registration, grades



Sales: customers, products, purchases



Online retailers: order tracking, customized recommendations



Manufacturing: production, inventory, orders, supply chain



Human resources: employee records, salaries, tax deductions

 Databases can be very large.
 Databases touch all aspects of our lives
Database System Concepts ­ 6th Edition

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©Silberschatz, Korth and Sudarshan

University Database Example
 Application program examples


Add new students, instructors, and courses



Register students for courses, and generate class rosters



Assign grades to students, compute grade point averages (GPA) and generate transcripts

 In the early days, database applications were built directly on top of

file systems

Database System Concepts ­ 6th Edition

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©Silberschatz, Korth and Sudarshan

Drawbacks of using file systems to store data


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 program code rather than being stated explicitly



Hard to add new constraints or change existing ones

Database System Concepts ­ 6th Edition

1.4

©Silberschatz, Korth and Sudarshan

Drawbacks of using file systems to store data (Cont.)





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 access needed for performance



Uncontrolled concurrent accesses can lead to inconsistencies
– Example: Two people reading a balance (say 100) and updating it by withdrawing money (say 50 each) at the same time



Security problems


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

Database systems offer solutions to all the above problems

Database System Concepts ­ 6th Edition

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©Silberschatz, Korth and Sudarshan

Data Models
 A collection of tools for describing

Data
 Data relationships
 Data semantics
 Data constraints


 Relational model
 Entity­Relationship data model (mainly for database design)
 Object­based data models (Object­oriented and Object­relational)
 Semistructured data model (XML)
 Other older models:



Network model
Hierarchical model

Database System Concepts ­ 6th Edition

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©Silberschatz, Korth and Sudarshan

Relational Model
 Relational model (Chapter 2)
 Example of tabular data in the relational model

Columns

Rows

Database System Concepts ­ 6th Edition

1.10

©Silberschatz, Korth and Sudarshan

A Sample Relational Database

Database System Concepts ­ 6th Edition

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©Silberschatz, Korth and Sudarshan

Data Definition Language (DDL)


Specification notation for defining the database schema
Example:
create table instructor ( ID char(5), name varchar(20), dept_name varchar(20), salary numeric(8,2))



DDL compiler generates a set of table templates stored in a data dictionary



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


Database schema



Integrity constraints


Primary key (ID uniquely
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