UGC NET CSE | January 2017 | Part 2 | Question: 18

Question

Let $M$ and $N$ be two entities in an $E-R$ diagram with simple single value attributes. $R_{1}$ and $R_{2}$ are two relationship between $M$ and $N$, where as $R_{1}$, is one-to-many and $R_{2}$, is many-to-many.

The minimum number of tables required to represent $M, N, R_{1}$, and $R_{2}$, in the relational model are ___________.

• A. $4$
• B. $6$
• C. $7$
• D. $3$

Comments

Total four table required.

M, (n,r1),n,(m,n,r2)

for R1 we don't need any table as we can put the relation on N side.

for R2 we need a separate table.

 

correct me if am wrong

Answer 1

Ans is option 4 i.e 3 relations

Two tables are created for entities M and N      and third table is created for relationships R2 (many to many)

for R1 no need of seperate table (one to many)

• 1-m relationships
  The primary key from the `one side' is placed as a foreign key in the `many side'.
• m-n relationships
  A new relation is created with the primary keys from each entity forming a composite key.
• 1-1 relationships
  Depending on the optionality of the relationship, the entities are either combined or the primary key of one entity type is placed as a foreign key in the other relation.

Answer 2

There will be two separate table for the two entities M and N.There will be a separate table for R1 relationship where primary key will be the primary key of the relation N.But R1 is one to many from M to N.So the relationship can be merged to the many side entity that is N.So there will be a single table for the entity N and relationship R1.For another relation R2 which is many-to-many there will be a separate table.

So total number of tables will be 3.

(Important points:

For many-to-many relationships there will always be a separate table and that table can not be merged with another entity.Primary key will be the combination of the primary keys of the participating entities).

Comments

Yeah option (D)3 is the right answer

Answer 3

Option 4-3 tables

M,NR1,R2

Answer 4

Minimum three tables will be required as table for M.NR1 & R2.