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

Option	Verdict	Reason
A. Every entity in E1 is associated with exactly one entity in E2	✅ True	Many-to-one + total participation from E1 guarantees this
B. Some entity in E1 is associated with more than one entity in E2	❌ False	That would be many-to-many, not many-to-one
C. Every entity in E2 is associated with exactly one entity in E1	❌ False	Could be associated with many E1s due to many-to-one
D. Every entity in E2 is associated with at most one entity in E1	❌ False	Again, many E1s can point to the same E2, so this is wrong

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you only needed to know total participation, which could be infered from the literal sense. Other than that it was a question that did not need DBMS knowledge and could be solved by Functions chapter knowledge.

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Student teacher analogy

We can solve this problem by thinking of two sets a STUDENT set and a TEACHER set.

Usually many student are assigned to a single teacher.

but all student gets a single teacher.

HENCE ,

1. Every entity in E1 is associated with exactly one entity in E2

Answer 1

Since it is a many to one relationship from E1 to E2, therefore:

1. No entity in $E1$ can be related to more than one entity in $E2$. ( hence B is incorrect)
2. An entity in $E2$ can be related to more than one entity in $E1$.(hence C and D are incorrect).

Option (A) is correct: Every entity in E1 is associated with exactly one entity in E2.

Comments

In a single line-Doble line representation, We write the cardinality on the opposite side of the relationship.can you tell if its followed here?I am literally confused in this question.

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@anmol_dalela That’s not required. If Cardinality Ratio of $E1:E2$ is $M:1$ it signifies many entries of $E1$ can relate with $1$ entry of $E2$. Hence $M:1$ is called many-to-one relationship. Double lines indicate total participation, hence every entity must be related.

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@Abhrajyoti00 How many relational table required here..? I think 1 table is sufficient with attribute on E1 as primary key right?

Answer 2

It is given that  "R is a many-to-one relationship from entity set E1 to entity set E2"

E1------^N-------R---------¹-------E2

Therefore,

Participation of E1:=1

Cardinality of E1:=1

 

Participation of E2:=1

Cardinality of E2:=N

 Therefore, Answer is (A): Every entity in E1 is associated with exactly one entity in E2 because the number of relationships in which an entity of a entity type E1 can participate is equal to 1.

 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.

Therefore, the line "cardinality of E1 is greater than the cardinality of E2" just means that the number of instances/entities in E1 is more than the number of instances/entities in E2

Comments

@kapilthukral94,  

1)Cardinality of E1:=1   & Cardinality of E2:=N

Since, it is N:1 relation, E1 can only be associated with only 1 element of E1, so Cardinality of E1:=1

and since, an element of E2 can be associated with many elements of E1, Cardinality of E2:=N.

Is this the correct way of understanding this ?

 

2)Participation of E1:=1   &  Participation of E2:=1.

I am still bit confused on this part. can u pls shed some light on it ?

 

Thanks in advance 

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Damn, I really needed to know the difference between them two. Thank you very much.

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this is the right explanation, thanks

Answer 3

Total participation means every entity is involved in relation.It is denoted by double lines.

Many to one relationship from Entity set E1 to Entity set E2 means no entity in E1 can be related to more than one entity of E2 but one entity of E2 can be related to more than one entity of E1. 

So,right answer is option A.

Comments

In the context of databases, cardinality refers to the uniqueness of data values contained in a column. High cardinality means that the column contains a large percentage of totally unique values. Low cardinality means that the column contains a lot of “repeats” in its data range.

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yaa, suppose if they give cardinality ratio for the relation is 1:N, then should we interpret it as :
'N' side contains more uniqueness and '1' side contains less uniqueness ??

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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.

Therefore, the line "cardinality of E1 is greater than the cardinality of E2" just means that the number of instances/entities in E1 is more than the number of instances/entities in E2

I mentioned cardinality 1:N with respect to Cardinality of an entity in a relationship.

Answer 4

Since total participation of both side And many- one is given then E1 must be merge with Relation.

So Every entity of E1 must be associated with exactly one entity in E2

A is answer

Comments

given cardinality greater

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Its given that cardinality of E1 is greater so E2 should be merged. And answer should be C

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If relationship is 1- m   then answer c possible

Answer 5

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

@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.