Cardinality Ratio concept in "Database and Management Systems" with explanatory figurative example

 

Cardinality Ratio concept in DBMS

 

Cardinality Ratio

It is the number of relationship instance that an entity can participate in . If one tries to understand the relationship between Student and Guide , then the relationship between the Student entity and the guide entity can be described in the following entity - relationship diagram .

 

Considering the given case , one can observe and try to understand the given relationship with the help of a Entity Relation Diagram with the help of a Set Diagram .

 

Here , some scenarios emerge like the given case :

Case-1

One guide (G1) can guide two students (S1) and (S2) whereas (S1) can only be guided by guide (G1) .

 

Case-2

Third Student (S3) can be guided by guide (G2) .

 

This is a kind of restriction set by the relationship where both the entity sets are mutually associated to each other by the relation between them . So here , the number of instance objects relation among each other is restricted and it can be also observed how the cardinality relationship is mapping one particular instance to another particular instance object through the given relationship diagram which depicts the way the entities can participate in .

From the above one can get a pretty good understanding of what a Cardinality Ratio is : -

Definition of Cardinality Ratio

The number of relationship instances that an entity can participate in is called as the Cardinality Ratio .

 

From the above diagram it can be noted that , one department would have only one HOD . So , in this case , the relationship would be only 1 is to 1 .

From the diagram we deduce that :

Relationship R1 exists from Department D1 to HOD H1 .

Relationship R2 exists from Department D2 to HOD H2 .

Relationship R3 exists from the Department D3 to HOD H3 .

 

A scenario over where such a type of relationship exists where there is only one relationship mapping from one Set's instance object to another Set's instance object is called a One to One relationship .

 

One can get a better understanding of this through the help of a E-R diagram shown at the bottom of the above figure .

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 The second type of relationship that exists is called One to Many Relationship

  

In the given figure , one can notice that there is a relationship existing between many departments and one student . Each department will have multiple students and thus one can notice from the given relation diagram that multiple relations exist from one department to student of another set but the student would be associated with only one department .

 This is an instance of Many to One Relationship which is depicted by the Ratio form of representation (1:M) .which is another form of cardinality ratio expressed in the form of Many to One relation .

 This means that Many Instances of any particular Entity Type will be associated or will be participating in the "Has" relationship .

Many to Many Relationship

In the given example , if one can see then one would be able to determine that there is Many to Many Relationship between the students set and Subjects set .

 

This can be rightfully depicted in the form , Relation between a Student on the left hand side and Subject on the right hand side of the Relationship Diagram . One can notice that multiple instance objects belonging to the set "Student" bear a many to many relationship between students of the other entity set which is the "Subject" set .

The depiction of the relation has been done through the help of an E-R diagram .

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Now , we can get to understand the behaviour of these relationships that depict the manner in which relationships exist between instance objects of one set with another or multiple other sets in the article's showcased manner