UNIT01-DBMS

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NOTES -QUESTION PAPERS

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DATABASE MANAGEMENT SYSTEMS

Contents

• Introduction to DBMS

• DBMS Users

• DBMS architecture

• Data Models

• ER Modeling

• Relational Model

• SQL

• Normalization www.BookSpar.com | Website for Students | VTU NOTES -QUESTION PAPERS

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What Is a DBMS?



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• Database : A very large, integrated collection of data.

• A Database Management System (DBMS) is a software package designed to store and manage databases.


Database System Applications

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Database Applications:

• Banking: all transactions

•

•

Airlines: reservations, schedules

Universities: registration, grades

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•

•

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Sales: customers, products, purchases

Online retailers: order tracking, customized recommendations

Manufacturing: production, inventory, orders, supply chain

Human resources: employee records, salaries, tax deductions

Databases touch all aspects of our lives



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Purpose of Database Systems

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•

•

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

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


Purpose of Database Systems (Cont.)

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•

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Drawbacks of using file systems (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 accessed needed for performance

• Uncontrolled concurrent accesses can lead to inconsistencies

• Example: Two people reading a balance and updating it 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

Why Use a DBMS?



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• Data independence and efficient access.

• Reduced application development time.

• Data integrity and security.

• Uniform data administration.

• Concurrent access, recovery from crashes.

Database Users



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•

•

•

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

Application programmers – interact with system through

DML calls

Sophisticated users – form requests in a database query language

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

• Examples, people accessing database over the web, bank tellers, clerical staff


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•

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Coordinates all the activities of the database system

• has a good understanding of the enterprise’s information resources and needs.

Database administrator's duties include:

• Storage structure and access method definition

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• Schema and physical organization modification

Granting users authority to access the database

•

• Backing up data

Monitoring performance and responding to changes



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View of Data

An architecture for a database system


Levels of Abstraction


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

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

type customer = record

customer_id : string;

customer_name : string;

customer_street : string;

customer_city : string;

end;

View level: application programs hide details of data types.

Views can also hide information (such as an employee’s salary) for security purposes.


Instances and Schemas


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• Similar to types and variables in programming languages

• Schema – the logical structure of the database

• Example: The database consists of information about a set of customers and accounts and the relationship between them)

•

•

• Analogous to type information of a variable in a program

Physical schema: database design at the physical level

Logical schema: database design at the logical level



Instances and Schemas

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

• Applications depend on the logical schema

• 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 Independence

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• Applications insulated from how data is structured and stored.

• Logical data independence: Protection from changes in logical structure of data.

• Physical data independence: Protection from changes in physical structure of data.



One of the most important benefits of using a DBMS!


History of Database Systems

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•

•

1950s and early 1960s:

• Data processing using magnetic tapes for storage

•

• Tapes provide only sequential access

Punched cards for input

Late 1960s and 1970s:

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•

•

• Hard disks allow direct access to data

Network and hierarchical data models in widespread use

Ted Codd defines the relational data model

•

• Would win the ACM Turing Award for this work

IBM Research begins System R prototype

• UC Berkeley begins Ingres prototype

High-performance (for the era) transaction processing



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History (cont.)

•

•

•

1980s:

• Research relational prototypes evolve into commercial systems

• SQL becomes industry standard

•

• Parallel and distributed database systems

Object-oriented database systems

1990s:

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•

• Large decision support and data-mining applications

Large multi-terabyte data warehouses

• Emergence of Web commerce

2000s:

XML and XQuery standards

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•

• Automated database administration

Increasing use of highly parallel database systems

Web-scale distributed data storage systems



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Data Models

•

A data model is a collection of concepts for describing data.

• Relational model

• Entity-Relationship data model (mainly for database design)

• Object-based data models (Object-oriented and

Object-relational)

• Semi structured data model (XML)

• Other older models:

– Network model

– Hierarchical model

The relational model of data is the most widely used model today.



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• A database can be modeled as:

• a collection of entities,

Database DesignConceptual design:

(

•

ER Model

) is an object that exists and is distinguishable from other objects.

•

•

• Example: specific person, company, event, plant

Entities have attributes

• Example: people have names and addresses

An entity set is a set of entities of the same type that share the same properties.

•

• Example: set of all persons, companies, trees, holidays

Relationship: Association among two or more entities. E.g., Aisha works in Pharmacy department

ER-diagram



24 ssn name

Employees lot

Covers policyid

Policies cost pname age

Dependents

Relational Model



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• Example of tabular data in the relational model

Attributes



A Sample Relational Database

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Logical DB Design: ER to Relational

• Entity sets to tables:

SSN Name lot ssn name

Employees lot



INTRODUCTION TO SCHEMA REFINEMENT

Problems Caused by Redundancy

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• Storing the same information redundantly , that is, in more than one place within a database, can lead to several problems:

• Redundant storage: Some information is stored repeatedly.

• Update anomalies: If one copy of such repeated data is updated, an inconsistency is created unless all copies are similarly updated.

• Insertion anomalies: It may not be possible to store some information unless some other information is stored as well.

• Deletion anomalies: It may not be possible to delete some information



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Bad DB Design to good DB Design

• Informal method

• Informal design guidelines

• Formal method

- Normalization

Normal Forms



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•

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If a relation is in a certain normal form (BCNF,

3NF etc.), it is known that certain kinds of problems are avoided/minimized. This can be used to help us decide whether decomposing the relation will help.

FDs are used to detect redundancy





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Example decomposition(simple)

• Bad database design

Eno Ename Salary Add Bdate Dno Dname Mngname

• Good database design

Eno Ename Salary Add Bdate Dno

Dno Dname Mngname


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Specification notation for defining the database schema

Example: create table account ( account_number char(10),

branch_name 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)

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•

Database schema

Data storage and definition language

• Specifies the storage structure and access methods used

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Integrity constraints

• Domain constraints

• Referential integrity (e.g. branch_name must correspond to a valid branch in the branch table)

Authorization



Data Manipulation Language (DML)

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• 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

• SQL is the most widely used query language

•

SQL



SQL : widely used non-procedural language

•

•

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Example: Find the name of the customer with customerid 192

select customer_name

from customer

where customer_id = 192

Example: Find the balances of all accounts held by the customer with customer-id 192

select balance

from depositor, account

where customer_id = 192 and depositor.account_number = account.account_number



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SQL

• Application programs generally access databases through one of

• Language extensions to allow embedded SQL

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


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1.

Parsing and translation

2.

Optimization

3.

Evaluation


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• Alternative ways of evaluating a given query

• Equivalent expressions

• Different algorithms for each operation

• Cost difference between a good and a bad way of evaluating a query can be enormous

• Need to estimate the cost of operations

• Depends critically on statistical information about relations which the database must maintain

• Need to estimate statistics for intermediate results to compute cost of complex expressions


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A transaction is 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) and transaction failures.

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



Transaction Concept

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• A transaction is a unit of program execution that accesses and possibly updates various data items.

• E.g. transaction to transfer $50 from account A to account B:

1.

read ( A )

2.

A := A – 50

3.

write ( A )

4.

read ( B )

5.

B := B + 50

6.

write ( B)

• Two main issues to deal with:

• Failures of various kinds, such as hardware failures and system crashes

• Concurrent execution of multiple transactions



Example of Fund Transfer

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Transaction to transfer $50 from account A to account B:

1.

read ( A )

2.

A := A

–

50

3.

write ( A )

4.

read ( B )

5.

B := B + 50

6.

write ( B)

Atomicity requirement

• if the transaction fails after step 3 and before step 6, money will be “lost” leading to an inconsistent database state

• Failure could be due to software or hardware

• the system should ensure that updates of a partially executed transaction are not reflected in the database

Durability requirement

— once the user has been notified that the transaction has completed (i.e., the transfer of the $50 has taken place), the updates to the database by the transaction must persist even if there are software or hardware failures.

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• Transaction to transfer $50 from account A to account B:

1.

read ( A )

2.

A := A – 50

3.

write ( A )

4.

read ( B )

5.

B := B + 50

6.

write ( B)

Example of Fund Transfer

(contd..)

• Consistency requirement in above example:

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• the sum of A and B is unchanged by the execution of the transaction

A transaction must see a consistent database.

During transaction execution the database may be temporarily inconsistent.

When the transaction completes successfully the database must be consistent

• Erroneous transaction logic can lead to inconsistency www.BookSpar.com | Website for

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QUESTION PAPERS



Example of Fund Transfer (Cont.)

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Isolation requirement — if between steps 3 and 6, another transaction T2 is allowed to access the partially updated database, it will see an inconsistent database (the sum A + B will be less than it should be).

T1 T2

1.

read ( A )

2.

A := A – 50

3.

write ( A ) read(A), read(B), print(A+B)

4.

read ( B )

5.

B := B + 50

6.

write ( B

Isolation can be ensured trivially by running transactions serially

• that is, one after the other.

However, executing multiple transactions concurrently has significant benefits.



ACID Properties

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A transaction is a unit of program execution that accesses and possibly updates various data items.To preserve the integrity of data the database system must ensure:

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•

•

Atomicity . Either all operations of the transaction are properly reflected in the database or none are.

Consistency .

Execution of a transaction in isolation preserves the consistency of the database.

Isolation .

Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions.

Intermediate transaction results must be hidden from other concurrently executed transactions.

• That is, for every pair of transactions T i finished execution before T i started, or T and j

T j

, it appears to T i that either started execution after T i

T finished.

j

,

Durability . After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures.


Transaction State


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Concurrent Executions

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• Multiple transactions are allowed to run concurrently in the system.

Advantages are:

• increased processor and disk utilization , leading to better transaction throughput

• E.g. one transaction can be using the CPU while another is reading from or writing to the disk

•

• reduced average response time for transactions: short transactions need not wait behind long ones.

Concurrency control schemes

– mechanisms to achieve ACID properties with concurrency.

• Concurrency control protocols are used in DBMSs to control concurrency


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UNIT01-DBMS

DATABASE MANAGEMENT SYSTEMS

Contents

What Is a DBMS?

Database System Applications

Purpose of Database Systems (Cont.)

Why Use a DBMS?

Database Users

View of Data

Levels of Abstraction

Instances and Schemas

Instances and Schemas

Data Independence

One of the most important benefits of using a DBMS!

History of Database Systems

History (cont.)

Data Models

A data model is a collection of concepts for describing data.

Database DesignConceptual design:

ER Model

ER-diagram

Relational Model

A Sample Relational Database

Logical DB Design: ER to Relational

Bad DB Design to good DB Design

Normal Forms

If a relation is in a certain normal form (BCNF,

3NF etc.), it is known that certain kinds of problems are avoided/minimized. This can be used to help us decide whether decomposing the relation will help.

FDs are used to detect redundancy

Example decomposition(simple)

Data Manipulation Language (DML)

SQL

SQL

Transaction Concept

Example of Fund Transfer

Example of Fund Transfer

Example of Fund Transfer (Cont.)

ACID Properties

Transaction State

Concurrent Executions

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