Mapping Cardinalities, Relationship, ER-Diagram

Mapping Cardinalities

In this tutorial, we shall discuss the Mapping Cardinalities, Relationships and ER-Diagram. There are four different types of mapping cardinalities. They are:

One to One:

 If we consider the two sets A&B. The entity in A associated with at most one entity in B and in the same way, one entity in B is associated with the at most one entity in A.

One to Many: 

In the consideration of two sets A&B. The entity in A associated with one or more than one entity in B, however every entity in B associated with at most one entity in A.

Many to One: 

The entity in A associated with at most one entity in B, but every entity in B is associated with one or more entities in A.

Many to Many:

An entity in A associated with one or more than one entity in B and similarly, the entity in B associated with one or more than one entity in A.

Relationship:

A relationship is an association among several entities.

Relationship Set:

A relationship set is a set of relations of the same type. It is a mathematical relation on n>=2 entity sets.

Entity 1 – Student(roll, name, age)

Entity 2 – Book(bkno, author, subject)

The relation can be between the student to the book taken from the library

Relation set can be –  issue(roll, bkno)

Having two entities in a relationship is known as binary relationship, three entities is known as ternary relationship and n entities is known as n-ary relationship

Entity – Relationship Diagram (ER Diagram)

ER diagram is a graphical representation of the logical structure of the database.

ER Diagram consists of the following components:

Rectangles – denotes the entities

Ellipse – denotes the attributes

Diamonds – denotes a relationship

Lines – denotes links between attribute, relationships and entities

Double Ellipse – denotes the multi-valued attribute

Dashed Ellipse – denotes derived attribute

Double Lines   – denotes total participation that is the merging between relationship and entity

Double Rectangles – denotes the weak entity set (set with a partial key without primary key)

For more information visit the section of SQL.