Database Systems

Database Systems

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Fundamentals of Database Systems (5th

Edition) Ramez Elmasri /Shamkant B. Navathe

Database Management Systems (2nd

Edition) - Raghu Ramakrishna

/Johannes Gehrke


Introduction

A Database is a collection of related data

Data: known fact that can be recorded and have implicit meaning

Eg : names, telephone numbers, student marks, etc


Introduction

Database Management System(DBMS) is a collection of programs that enables

users to create and maintain databases.

Database System = database + DBMS


File Systems

Before Mid 60s

Operating Systems files provide the basic means of storing information.

Files are sufficient for applications requiring small amounts of data &

mainly single-user applications.


Database Systems vs File Systems

Problems with file systems

Data redundancy

Integrity issues

Required knowledge of storage details

No standard language

Files only provide sequential access

Basic security capabilities (read, write, execute)


Evolution of Database Models

First Generation

Hierarchical & Network Model

Second Generation

Relational Model

Third Generation

Object Relational Model

Object Oriented

XML-DB


Evolution of Database Models

In early 1960s, Charles Bachman proposed the Network Data Model

In the late 1960s, IBM developed Information Management System (IMS) based on the Hierarchical Data Model

In 1970, E.F. Codd of IBM - San Jose Research Laboratory, proposed the Relational Data Model


Relational Database Model

Relation can be thought as a table of values

Each row in the table represents a collection of related data

Row is called a tuple

Column header is called an attribute

Table is called the relation


ANSI SPARC 3 Level Architecture

External schema

Conceptual/Logical schema

Internal/Physical schema

Mapping

External conceptual mapping

Conceptual physical mapping

Data independence

Logical

Physical


ANSI SPARC 3 Level Architecture


External Schema

Describe the database as it is seen by the user, and the user applications.

Describe the part of the database that a particular user group is interested in and

hides the rest of the database


Conceptual/Logical Schema

Describe the structure of the whole database


Internal/Physical Schema

Describes the physical storage structure of the database


Mapping

The process of transforming requests and results between levels

There are two types of mapping. External-Conceptual Mapping

Conceptual-Internal Mapping


Data Independence

The capacity to change the schema at one level of a database system without

having to change the schema at the

next higher level

There are two types of data independence

1.Logical data independence

2.Physical data independence


DATABASE DESIGN PROCESS


Database Designing Steps

Requirements Analysis

Conceptual Database Design

Logical Database Design

Schema Refinement

Physical Database Designing

Security Design


Conceptual Database Design

In this step, a high-level conceptual model is designed encapsulating the data requirements

Entity-Relationship (ER) Model is a very popular & widely used conceptual model


Entity Relationship Model

An entity is an object in the real world

A collection of similar entities is called an entity set.

Graphically,


ENTITY


An entity is described using a set of attributes.

Graphically,

For each attribute, a domain of possible values for the attribute needs to be

defined


Attribute


Attributes containing multiple values are called multivalued attributes.

Graphically,


EMPLOYEE

first_nameother_names

surname


Composite Attributes can be divided in to smaller subparts

Graphically,


Name

First

Middle

Last


Complex attributes

Derived attributes and stored attributes


Relationship

A relationship is an association among two or more entities

A relationship can have descriptive attributes used to record information about the relationship


Relationship name

Relationship name

Descriptive Attribute

Relationship

Participation constraint specifies the fact that the relationship is required for

every entity of the entity set.

The relationship set has either total or partial participation

Graphically, bold line represents a total participation


Degree of a Relationship is the number of participating entity types.

A relation type of degree two is called binary, and one of degree three is

called ternary


The cardinality ratio for a binary relationship specifies the number of

relationship instances that an entity can

participate in.

There are three types of cardinality ratios for binary relationships. They are

one-to-one, one-to-many (many-to-

one) and many-to-many.


1:1

1:M

M:1


works

inEMPLOYEE DEPARTMENT

works


works


1 1

1 M

M 1

M:M


works


M N

Logical Database Design

ER Relational Mapping


Relational Model

The basic construct of the relational model is the relation.

A relation consists of

relation schema

relation instance


Relation Schema = relation name + {name of each field: domain of each

field}

COURSE (course_no: integer, title:

string, credits: integer, year: integer,

semester: integer)

Relation Instance = set of tuples


A relational database is a collection of relations with distinct relation names

Integrity constraint is a rule (or check) that is enforced by the DBMS to ensure

that incorrect data is not stored in the

database


Domain constraints

Key Constraints

Entity integrity constraints

Referential integrity constraints

Other constraints


Domain Constraints

Domain constraints: specify that each attribute A must be an atomic value from the

domain of A.

Atomic value: That is, the value is non divisible into components

Therefore, relational model doesnt allow multi-valued attributes or composite attributes!

Relational database provides data types to specify valid domains.


Key Constraints

Super key Suppose that we denote one such subset of

attributes by SKi

For any two distinct tuples t1 and t2 in a relation state r of R

t1[SK] t2[SK]

SK is called a superkey of the relation

Specifies a uniqueness constraint

Every relation has at least one default superkey


Key Constraints

Key A key K of a relation schema R is a superkey of

R

Removing any attribute A from K leaves a set of attributes K' that is not a superkey of R any

more

It is a minimal superkey

A superkey from which we cannot remove any attributes and still have the uniqueness constraint

Eg:

{SSN, Name, Age} is a superkey but not a key

The property is time-invariant:


Key Constraints

Candidate key A relation schema may have more than one

key

Each key is called a candidate key

Primary key A candidate key

values are used to identify tuples in the relation

Better to choose a primary key with a single attribute or a small number of attributes

The primary key of a relation schema are underlined


Key Constraints

Alternate key In the candidate key the key which is not

primary key is the alternate key.


Entity Integrity Constraints

No primary key value can be null

This is because the primary key value is used to identify individual tuples in a

relation


Referential Integrity Constraints

Specified between two relations

Used to maintain the consistency among tuples in the two relations.

A tuple in one relation that refers to another relation must refer to an

existing tuple in that relation


ER to Relational Mapping

Entity

Simple Attributes

Primary Key

Composite attributes

Relationships

1:1

1:M or M:1

M:N

Nry


ER to Relational Mapping

Descriptive attrbutes

Multivalued attributes


SCHEMA REFINEMENT


Conceptual Modeling is a subjective process

Therefore, the schema after the logical database design phase may not be very good (contain redundancies)

However, there are formalisms to ensure that the schema is good.

This process is called Normalization


STRUCTURED QUERY

LANGUAGE (SQL)


Data Definition Language

The main SQL command for data definition is the CREATE statement

Can be used to create schemas, tables, domains, etc


Data Definition Language

CREATE TABLE My_table(

my_field1 INT,

my_field2 VARCHAR(50),

my_field3 DATE NOT NULL,

PRIMARY KEY (my_field1, my_field2)

);

ALTER TABLE My_table ADD my_field4

NUMBER(3) NOT NULL;

TRUNCATE TABLE My_table;

DROP TABLE My_table;


Data Control Language

GRANT - gives user's access privileges to database

REVOKE - withdraw access privileges given with the GRANT command


Data Control Language

GRANT SELECT, UPDATE ON My_table TO some_user, another_user;

REVOKE SELECT, UPDATE ON My_table FROM some_user,

another_user;


Data Manipulation Language

Select

Insert

Update

Delete


Database Systems

Documents

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