Lesson 20 ER diagram

Lesson 19 Aggregation

Aggregation is the process of compiling information on an object, there by abstracting a higher level object. One limitation of E-R model is that it cannot express relationship with in relationship to illustrate the need for such a construct the ternary (compose of three) relationship work_on, between employee branch job so in figure. Thus the relationship set work_on relating the entity set, such an entity set is treated in the same manner as in any other entity set. We can then created a binary relationship manager to represent who manage what task

Lesson 18 What is class and object

What is class?

Class is a set of object that share common structure and common behaviour is called class.

For example

Flora, fauna, flower, animal, bird etc..

What is object?

An object is something that has a fixed set of well define boundary. If you look at your computer disk you would object varying descriptions

For example

A keyboard may be attached to pc and it is also easy to see different parts of pc are distinct objects

&Note

In ordinary sense, an object is something which can be seen, touch or sensed.

Lesson 17 Inheritance

Inheritance means that the behaviour and data associated with child classes are always an extension of the properties associated with present class. A subclass must have all the properties of the present class and other properties as well.

Lesson 16 Generalization

Now consider the opposite of above example. Two entities saving account and current account have some attribute that are common. In this case, a better option is to create a new separately to simplify multiple references. This is called generalization.

&Note

Generalization is the result of taking the union of two or more lower level entity set to produce higher level entity set.

Lesson 15 Specialization

Specialization is result of taking subject of higher level entity set for lower level entity set.

Lesson 14 Common attributes

Attributes may entities

Lesson 13 Subtype and super type

A sub type is subset of another entity. A subtype is always dependent on super type for its existence.

Lesson 12 Strong & weak entity

Database design using E-R Model

A database which conforms to an ER diagram can be represented by a set of tables.

Strong & weak entity

These are two type of entity mainly dependent entity (also called weak entity) and independent entity also (called regular entity and weak entity).

The dependent entity is the one whose existence dependents on another entity.  An entity set is called weak entity set if its existence depends on the existence of another weak entity or any other strong entity. A weak entity set does not have sufficient attributes to form a primary key.

Note => A weak entity set represented by double outline rectangle in ER diagram.

Consider an entity type marks which represent the marks obtained by student. Now the existence of marks dependence upon the existence of entity type student.

&Note

A weak entity set can be converted to strong entity set by adding appropriate attributes.

Let’s take real life situation to understand type of entity. Suppose a student says ‘Swati’ can have many instance of marks say eng_marks; sci_marks; sst_marks etc. related to it if instance ‘Swati’. If Swati is deleted then all the entity marks dependent upon swati will also removed.

Marks is dependent (weak) entity that is dependent on entity student.

For example

The entity Employee & Student are strong entity they have the primary keys Emp_no & admin no 

&Note

An entity set which has primary key termed as a strong entity set

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Lesson 11 Why we use Entity Relationship diagram ERD

"Many of my students ask me what is ERD? Why ERD is used? What the use of studding DBMS.... so on and on and on "

"They are actually true. What is the use of this Subject. If you say its too late to explain this I would be post this dialog early posting some where."

"You may be right but in my point of view you know nothing about this Topic. Now we are moving to advance level actually its is the best time to chat with you guys. So let me explain.."

what is dbms actually all about.

I have good example...

My dear friends you are reading my blog. So you Know about computer programming as well Let us assume that.. well i am sure about it!

Then you surely heard about algorithms in programming.
Algorithms are some thing like frame setup for an complex programming.
simply in Alogo (aka) we decide how to develop a program, whats the structure of the program. how many veritable are used in a program. what approach we should take to solve the problem.

ERD is an algorithms in software development or designing. Let me explain more clearly by an other example.

Assume You are the Owner of a software development company Let's Name it Sam enterprises 

Now I come to you having a small school. I want to upgrade my school. hand work to computerization.

If you know only programming You are not able to handle it.

So Entity relationship model is just a Blueprint of a client organization..

you better understood later lessons.

Lesson 10 Symbols of E-R Model

Lesson 10

Symbols of E-R Model

Lesson 9 Key Attribute

  

      Key Attribute

Key attribute is a uniquely identified in the entity set.

For example

Employee can be the key attributes of the entity set. Employee because it uniquely identified is employee is employee entity

Lesson 8 Many to Many M:M

Lesson 8

c) Many to Many (M:M)
A many to many relationship describe entities that may have many relationship among each other

For example
I. One customer may buy many items and one item may be bought by many customers.
II. A student can take many courses in university and many students can register for given course.

Lesson 7 One to many 1:m

      b) One to many (1:m)

One to many relationships exists when one entity is related to more than one entity. For example:-

1.      Father may have many children but a child have only one father.

2.      Two entities set namely courses & teacher, if you assume that more than one Subject or course is taught by a teacher, and then a relationship is one to many between teacher & courses.

Lesseon 6 One to One

Three type of relationship exist among entities
These are:
a) One to one (1:1)
b) One to many (1:m)
c) Many to many (m:m)

a) One to one (1:1)
A one to one (1:1) relationship is an association only between two entities.
For example, in a university each department has only one head of department. More over one faculty member cannot be the head of more than one department.
This shows one to one relationship between departments as head.
Another example of such relationships is between automobile & engine. If we assume that a car does not have two engines and that car must be filter with one engine in car.

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Lesson 5 Relationship

3. Relationship

To understand what we mean by relationship among data item, we need some familiarity with data model terminology. An entity is a thing which can be easily identified.
It may be an object, place, person, concept or activity for which data need to be recorded.entity of student instructor

Note 
An association among entity lead to relationship.


Example
There is some relationship between instructor and student as seen in figure

This relationship respect the fact that instructor teaches several student, and a student taught by several instructors.

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Lesson 1 INTRODUCTION TO DATABASE MANAGEMENT SYSTEM

Introduction

Database is an organized collection of fact. In other word we can say that it is collection of information arranged and presented to serve and assigned purpose.

An example of Database is a dictionary, where word arranged alphabetically. Another example is telephone directory.


Note
Thus information is stored and arrange in a particular manner

In order to keep database update we may need to perform operations like:-

• Adding information
•  Removing information
•  Editing the existing information

Why database in used?

Any organization be it a bank, manufacturing company, hospital, university, government department; required huge amount of data in same form or the other all such organization need to collect data, manipulate them and store them for future use.

Characteristics of Database

Fields: The smallest piece of meaningful information of file is called a database item or field. A database item is generally use for a group alpha numeric character.

A data item is generally used for a group of alpha numeric character figure display the data item, fields in records.

NAME, LOCATION, CITY, PIN CODE

Records: The collection of related field is called records.

Files: A collection of record is called files

Next Lesson 2    ENTITY RELATIONSHIP MODEL

Lesson 4 Attribute

2.Attribute 

Each property or attribute show in the ER diagram map to an attributes in the appropriate time (refer to the following figure). Properties or attributes of STUDENT & BOOKS map to attributes in the relevant table

>Note 

Remember that the primary key must be able to uniquely identify each tuple in table.

Lesson 3 Mapping E-R diagram to table

Mapping ER diagram to table
A database that conforms to an ER diagram can be represented by a collection of table in the relational system.


1. Regular Entity
2. Attributes
3. Relationship
4. Weak Entity
5. Subtype & super type 

1. Regular Entity You will recall entity are dependent, they can exist in isolation, independent of any other blocks of database. Each regular entity maps to table.

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Lesson 2 Entity Relationship Model

The Entity Relationship Model consist of the following component

a) Entity
b) Attributes 
c) Relationship 
d) Key Attributes

a) Entity 
An entity is class object event or concepts about which we need collect and store data.

• note

Entity is a thing which can be easily identified. An entity is any object, place, person or activity about which data is recorded.

• Person: - Employee, Student, Vender….
• Place: - Branch, Office, Building, room….
• Object: - Book, Machine, Pen…..
• Event: - Sell, Purchase, Registration…

b) Attributes An attributes is a property of given entity.

roll number is a property of an entity student.

NOTE
In dbms there are many words for a same thing

means as above i said field and then I said entity, some time its called columns, DON'T GET PUZZLED IN THAT
this is only the way some kind of differentiating different environments 
for example ROWS, RECORDS and TUPLES 
If you talking in RDBMS (Relational Data Base Management System) we called them TUPLES.
If you talking in DBMS (Data Base Management System) we called them RECORDS.
If you talking in MS EXCELL we called them ROWS.

=========================================
Column, Fields, Attributes

Column :- In EXCELL
Fields:- In DBMS
Attributes:- in E-R DIGRAM 
=======================================
Entity, Table, File

Entity:- In E-R DIGRAM
Table :- When we create/edit an ENTITY in form of Table.
File :- when data stored in hardware (hard disk)


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