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3 votes
3 votes

Let $pk(R)$ denotes primary key of relation $R$. A many-to-one relationship that exists between two relations $R_1$ and $R_2$ can be expressed as follows:

  1.  $pk(R_2)\rightarrow pk(R_1)$
  2.  $pk(R_1)\rightarrow pk(R_2)$
  3. $pk(R_2)\rightarrow  R_1 \cap R_2$
  4. $pk(R_1)\rightarrow  R_1 \cap R_2$
in Databases recategorized
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4 Answers

3 votes
3 votes
Best answer

Answer: $B$

$Pk(r)$ denotes the primary key attribute of relation $r$.

• The functional dependency $Pk(r_1) \rightarrow  Pk(r_2)$ indicates a many-to-one relationship between $r_1,r_2,$ since any $r_1$ value which is repeated will have the same $r_2$ value, but many $r_1$ values may have the same $r_2$ value.

NOTE:

Consider the statement:

  1. A many-to-one relationship set exists between entity sets student and instructor.
  2. A many-to-one relationship set exists between entity sets instructor and student.

Read the above statements & think about what do they mean.

Statement 1 says that “one instructor may have many students, but one student learns from at most one instructor”.

Statement 2 says that “one student may have many instructors, but one instructor teaches at most one student”.


Source of this UGC NET Question: Korth DBMS Book.

https://www.db-book.com/db6/practice-exer-dir/8s.pdf   (Page 10)

http://web.cs.ucla.edu/classes/fall04/cs143/solutions/ch7.pdf (Page 86)

Same Question here:

https://gateoverflow.in/225819/ugc-net-cse-july-2018-part-2-question-67 

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1 comment

option B?
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4 votes
4 votes

For example consider the relation R1 be employee (primary key eid} R2 be department (primary key did)

The many to one relationship is many employees works in one department

given an employee we can determine his department

given  a department there are n number of possilbe employees. so we cannot uniquely determine 

now based on definition of functional dependency we can conclude

pk(R1) → pk(R2)

3 votes
3 votes

Ans will be 2) pk(R1)→pk(R2)

Consider the following case 

A many-to-one relationship set  exists between entity sets   students  and course (a relation r with primary key roll_no )

let it is decomposed in 2 relations r1(student with Pk roll_no ) and r2 (course with Pk course name )

Roll_no can uniquely identify course name   but course  name can not uniquely identify roll_no 

so the functional dependency Pk(student) → Pk(course) indicates a many-to-one relationship between r1 and r2 

3 Comments

Can you please elaborate this sentence:

 

Roll_no can uniquely identify course name   but course  name can not uniquely identify roll_no
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Students and course is a many-to-many relation not many-to-one.
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Sir i think its many to one means R1 is in many side and r2 is one side. Therefore option 1 will be correct.
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0 votes
0 votes

i think the answer is C

A many to one relationship exists between two relations R1 and R2. Eg: R1 can be Subject table while R2 can be Student table and they can be related as one student studies many subjects.

Hence primary key (R2) specifies  R1∩R2

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1 comment

@sh!va not getting "Hence primary key (R2) specifies  R1∩R2"

could you plz elaborate some more..

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Answer:

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