GATE CSE 2018 | Question: 11

Question

In an Entity-Relationship (ER) model, suppose $R$ is a many-to-one relationship from entity set E1 to entity set E2. Assume that E1 and E2 participate totally in $R$ and that the cardinality of E1 is greater than the cardinality of E2.

Which one of the following is true about $R$?

• A. Every entity in E1 is associated with exactly one entity in E2
• B. Some entity in E1 is associated with more than one entity in E2
• C. Every entity in E2 is associated with exactly one entity in E1
• D. Every entity in E2 is associated with at most one entity in E1

Comments

This line makes confusion 

the cardinality of E1 is greater than that of E2.

 

Note: "Cardinality of a set/Entity set" is a different thing and "Cardinality of an Entity in a relationship" is a different thing.

Cardinality of a set/Entity set: Number of instances/elements in that set.

Cardinality of an Entity in a relationship: Maximum number of relationships in which an entity of a given entity type can participate.

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thanks! this explanation is enough to resolve all doubts related to this question.

Answer 1

Given that :-

(1) R is a many-to-one relationship from entity set E1 to entity set E2.

(2) Assume that E1 and E2 participate totally in R

(3) The cardinality of E1 is greater than the cardinality of E2.

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(B)  Some entity in E1 is associated with more than one entity in E2

         therefore many instances of E2 should be mapped with one instance of E1. So, R is one to Many

         Relationship But given that R is Many to One Relationship ===> Option (B) False

(C)  Every entity in E2 is associated with exactly one entity in E1

          according to (3), there are some instances of E1 should not mapped with any instance of E2

           So E1 should be partially participated in R. But given that E1 is participated totally in R ==>

          Option (C) False

(D)  Every entity in E2 is associated with at most one entity in E1

        therefore some of the E2 instances not mapped with E1. So E2 is partially participated But given that E2 is participated totally in R ===> Option (D) False

Comments

Well Explained.

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@Shaik Masthan there is a contrast between (1) & (3), I can't understand how both of these two are possible at a same time?????

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@MRINMOY_HALDER

why there is a contrast between (1) & (3) 

check this image, (1) and (3) with (2)

check this image, (1) and (3) without (2)

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@Shaik Masthan what is the meaning of cardinality here, is it the no of enity present in entity set or maximum participation in relation of entity set.

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what is the meaning of cardinality here, is it the no of enity present in entity set

yes...

 

what is the meaning of cardinality here, is it maximum participation in relation of entity set.

no... " R is a many-to-one relationship from entity set E1 to entity set E2 " conveys that maximum participation in relation of entity set.