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Data on a Touch Pad

RDBMS FUNDAMENTALS

E-R MODEL

•    A model that is used to represent entities and their relationship is called E-R model.
•    It stands for Entity-Relationship model.
•    An E-R model uses different symbols to represent entities, attributes and relationship.

•    Example:-

ER1.JPG

Elements of E-R model

•    The following are various elements of E-R model:-
    1.    Entity
    2.    Attribute
    3.    Relationship
    4.    Identifier

Entity

o    An entity is a real world object that has physical or logical existence.
o    It may represent a boy, a girl, a fan, a man, etc.
o    It is represented by a rectangular symbol with the name of entity written in capital letter.
o    Syntax:- 

entityname.JPG

o    Example:-
​

std.JPG

o    An entity has two parts:-
    I.    Entity instance:-
        It represents a part of an entity.
    II.    Entity type:-
        It represents the common behavior among entity instances.


o    Types of entity:-
    There are 2 types of entity:
    I.    Strong entity:-
        •    An entity that does not depend on other entities is called as strong entity or super entity.
        •    A strong entity must have an attribute that can be used as primary key.
        •    It is represented by rectangular symbol.
        •    Example:-

clg.JPG

    II.    Weak entity:-
        •    An entity that depends on other entities is called weak entity or sub entity.
        •    It may not have a primary key.
        •    It is represented by double rectangular symbol.
        •    Example:- 
 

weakentity.JPG

Attribute

o    Attributes are the properties of an entity.
o    The one or more fields that are used to describe an entity are called as attributes.
o    It is represented by oval shape.
o    Syntax:-

attr.JPG

o    Example:-
​

name.JPG

o    Types of attribute:-
    I.    Single valued attribute/simple attribute:-
        •    An attribute that can store only one value at a time is called as single valued attribute/simple attribute.
        •    Example:- roll, name, etc.
    II.    Multi-valued attribute:-
        •    An attribute that can store more than one value at a time is called as multi-valued attribute.
        •    Multi-valued attributes are represented by double oval shape.
        •    Example:- 

add.JPG

    III.    Composite attribute:-
        •    An attribute that can be decomposed into sub-attributes is called as composite attribute.
        •    Example:- name, address, etc.
    IV.    Stored attribute:-
        •    It is an attribute that has its own value.
        •    It means it has its own existence.
        •    Example:- salary, mark, etc.
    V.    Derived attribute:-
        •    It is an attribute which can be derived from stored attribute.
        •    It is represented by dotted oval shape.
        •    Example:- total_mark
 

Relationship

o    A relationship is the link between two or more entities.
o    It represents the common attribute or behavior among entity instances.
o    It is represented by a diamond symbol.
o    Types of relationship:-
    There are following types of relationship:-
    I.    One-to-one relationship:
        •    In one-to-one relationship, an instance of an entity links with a single instance of  an another entity.

stdhas.JPG

    II.    One-to-many/many-to-one relationship:
        •    In one-to-many relationship, an instance of an entity links with number of instances of another entity.
        •    The reverse of one-to-many relationship is called as many-to-one relationship.

enrolled.JPG

    III.    Many-to-many relationship:
        •    In many-to-many relationship, two or more instances of an entity may link with two or more instances of an another entity.

enrld1.JPG

Identifier

o    An identifier is one or more attributes that can uniquely identify an entity.
o    It can be represented by an underline symbol with the attribute.
o    All the attributes may not be identifiers but all the identifiers must be attributes.

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