UNIT01-DBMS

DATABASE MANAGEMENT SYSTEMS

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Contents

• Introduction to DBMS• DBMS Users• DBMS architecture• Data Models• ER Modeling• Relational Model• SQL• Normalization

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What Is a DBMS?

• 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 • Database Applications:

• Banking: all 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 touch all aspects of our lives







Purpose of Database Systems

• 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.)

• 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?

• Data independence and efficient access.• Reduced application development time.• Data integrity and security.• Uniform data administration.• Concurrent access, recovery from crashes.


Database Users

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


Database Administrator

• 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• Schema and physical organization modification• Granting users authority to access the database• Backing up data• Monitoring performance and responding to

changes


View of Datawww.BookSpar.com | Website for Students | VTU

NOTES -QUESTION PAPERS

An architecture for a database system

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Levels of Abstraction

• 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 = recordcustomer_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

• 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

• 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 • 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!

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History of Database Systems• 1950s and early 1960s:

• Data processing using magnetic tapes for storage• Tapes provide only sequential access

• Punched cards for input• Late 1960s and 1970s:

• 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


History (cont.)• 1980s:

• Research relational prototypes evolve into commercial systems• SQL becomes industry standard

• Parallel and distributed database systems• Object-oriented database systems

• 1990s:• Large decision support and data-mining applications• Large multi-terabyte data warehouses• Emergence of Web commerce

• 2000s:• XML and XQuery standards• Automated database administration• Increasing use of highly parallel database systems• Web-scale distributed data storage systems


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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Database Design- Conceptual design: (ER Model)

• A database can be modeled as:• a collection of entities,• relationship among entities.

• An entity 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


agepname

DependentsCovers

name

Employees

ssn lot

Policies

policyid cost

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Relational Model

• Example of tabular data in the relational model


Attributes

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A Sample Relational Databasewww.BookSpar.com | Website for Students | VTU

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Logical DB Design: ER to Relational• Entity sets to tables:


Employees

ssnname

lot

SSN Name lot

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INTRODUCTION TO SCHEMA REFINEMENTProblems Caused by Redundancy

• 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


Bad DB Design to good DB Design• Informal method

• Informal design guidelines

• Formal method

- Normalization


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


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Example decomposition(simple)• Bad database design

• Good database design


Eno Ename Salary Add Bdate Dno

Dno Dname Mngname

Eno Ename Salary Add Bdate Dno Dname Mngname

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Data Definition Language (DDL)• 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)• Database schema • Data storage and definition language

• Specifies the storage structure and access methods used

• 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)

• 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

• Example: Find the name of the customer with customer-id 192

select customer_namefrom customerwhere customer_id = 192

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

select balancefrom depositor, accountwhere customer_id = 192 and depositor.account_number =

account.account_number


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


Query Processing

1.Parsing and translation2. Optimization3. Evaluation


Query Processing (Cont.)

• 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


Transaction Management

• 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• 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• 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.


Example of Fund Transfer (contd..)

• Transaction to transfer $50 from account A to account B:1.read(A)2.A := A – 503.write(A)4.read(B)5.B := B + 506.write(B)

• Consistency requirement in above example:• 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

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Example of Fund Transfer (Cont.)• 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 T21. read(A)2. A := A – 503. write(A)

read(A), read(B), print(A+B)4. read(B)5. B := B + 506. 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

• 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 Ti and Tj, it appears to Ti that either Tj, finished execution before Ti started, or Tj started execution after Ti finished.

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


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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Transaction State www.BookSpar.com | Website for Students | VTU


Concurrent Executions• 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


Overall System Structure www.BookSpar.com | Website for Students | VTU


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UNIT01-DBMS