MG/DB CONCEPTS/CL12

DATABASE CONCEPTS

DATABASE

An electronic filing cabinet

A repository for a collection of computerized data files

A collection of persistent data that is used by the application system of some given enterprise

The term enterprise is a generic term for any self-contained commercial, scientific, technical or other organization

MG/DB CONCEPTS/CL12

DATABASE SYSTEM

A computerized record-keeping system. A database system involves four major components: data, hardware(secondary storage devices, processor(s) and associated main memory etc.,), software(the DBMS) and users(application programmers, end users, database administrator).

MG/DB CONCEPTS/CL12

Data independence

The ability to modify the scheme definition in one level without affecting the scheme definition in the next higher level is called data independence. Physical data independence :

refers to the ability to modify the scheme followed at the physical level without affecting the scheme followed at the conceptual level i.e., the application programs remain the same even though the scheme at physical level gets modified.

Logical data independence :

refers to the ability to modify the conceptual scheme without causing any changes in the schemes followed at view level.

MG/DB CONCEPTS/CL12

Data models

The hierarchical model

The network model

The relational model

MG/DB CONCEPTS/CL12

The hierarchical modelThe oldest model and is popular on main frames

Represents data by a collection of records organized in the form of a tree structure

Relationships among data are represented by links

The record at the top of the tree is the root

The hierarchy is maintained in the form of nodes and branches

The root may have any number of dependents, each of these dependents may have any number of lower level dependents and so on to any number of levels

MG/DB CONCEPTS/CL12

Consider a student course-marks database.

In the hierarchical model, a student can register for many courses and get marks for each course.

Scode SnameScode Sname

Ccode Cname Marks

C1 Physics 65

C2 Chemistry 78

C3 Maths 83

C4 CompSc 85

S2 BS1 A

Ccode Cname Marks

C3 Maths 83

C4 CompSc 85

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The student record is called as root.

It has got a course-marks record that is called the child record.

A parent can have many children.

A child cannot have more than one parent and no child can exist without its parent.

In this type of model following possibilities exist.Insert : Since no child record can exist without its parent, it is not possible to insert a new course details without introducing a dummy student

Delete : If a course is selected by only one student, then deleting that student will automatically delete all info about the course

Update : To change the course name of one course, the whole database has to be searched. This may result in data inconsistency.

MG/DB CONCEPTS/CL12

The network model

A NW model represents data by a collection of records and relationships among data are represented by links.

The student and course records are linked together through a marks record.

There are no restrictions on number of parents.

A record type can have any number of parent and child record types.

It is more complex than the hierarchical model because of links.

It can represent any structure designed in hierarchical model, and so is a superset of the hierarchical model.

MG/DB CONCEPTS/CL12

In network model following possibilities exist:

Insert

Inserting a course record or student record poses no problems as they can exist without any connectors till a student takes the course.

Delete

Deleting any record automatically adjusts the chain.

Update

Update can be done only to a particular child record

MG/DB CONCEPTS/CL12

The relational model

Developed by Codd

In relational model, data is organized into tables consisting of rows and columns

The table is referred to as a relation, the rows as tuples/records and columns as fields/attributes

A row in a table represents a relationship among a set of values

Since a table is a collection of such relationships, it is referred to as a relation

The relationships among data are represented by a collection of tables.

MG/DB CONCEPTS/CL12

Student tableMarks table

Scode Sname

S1S2

AB

Ccode Scode Marks

C1C2C3

S1S1S1

657883

C4C3C4

S1S2S2

858385

Course table

Ccode Cname

C1C2C3

PhysicsChemistryMaths

C4 Compsc

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Characteristics of a relation

There are no duplicate tuples

Tuples are not ordered

Attributes are not ordered

All attribute values are atomic

MG/DB CONCEPTS/CL12

Terminology

Cardinality

The number of tuples is called the cardinality

Degree

The number of attributes is called the degree

Domain

A pool of values from which specific attributes of specific relations draw their values.

A domain is just a data type

It provides a set of values from which various attributes in various relations take their actual values.

MG/DB CONCEPTS/CL12

Primary key

A column or a group of columns in a table that uniquely identifies records in the table.

It is a unique identifier for the table.

It contains unique value for each record.

Foreign key

A column or a group of columns in a table that is used to reference a primary key in another table.

MG/DB CONCEPTS/CL12

Database concepts (Revision)A database is a container for data

It is repository for storing organized information in the form of files

A db management system is a software package that handles the database

It is an interface between the user and the database

A DBMS organizes data as a data model

Data model is an infrastructure of data organization

MG/DB CONCEPTS/CL12

In a Relational Data Model, the data is organized into tables consisting of rows and columns

A table is referred to as a relation

The rows are referred to as tuples/records

The columns are referred to as fields / attributes

Primary key

A primary key is a unique identifier for the table

It consists of one/more attributes that uniquely identify each and every record in a table

It contains unique value for each record

MG/DB CONCEPTS/CL12

A database is a collection of tables

Tables consist of organized related information

Each table consists of a collection of records

Each record contains info about a specific entity

The info in each record is contained in fields

A field is the basic unit of a db

A field can contain text, numbers, dates or even photographs

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A data base system has 2 parts

The back end

The frond end

Front-end s/w interfaces with the user

Eg> Visual Basic

The back-end s/w holds data

Eg> IBM DB2, MS SQL SERVER, ORACLE, SYBASE, INFORMIX, MS ACCESS

MG/DB CONCEPTS/CL12

Structured Query Language(SQL) Enables you to create and operate on relational databases

A relational database is a collection of tables

Used for defining and manipulating relational databases

Non-procedural language

Format free language (Tabs, carriage return and spaces can be included anywhere in the statemen

Case insignificant

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The language consists of the following components:

Data definition component/language:

This component is used for defining and destroying objects like relations/tables, index etc

Eg> Create, alter, drop

Data manipulation component/language:

This component is used to retrieve and modify data.

It contains retrieval, manipulation and update directives

Eg> select, insert/delete, update

MG/DB CONCEPTS/CL12

Data control component:

Used for defining security and access control of the database.

Allows the user to grant access privileges.

Eg> grant, revoke