ISRO CS 2025 Q.46: solution and answer

Official answer: DSource: ISRO's final answer key of 15 May 2026

1. Each entity set with only simple, single-valued attributes maps to one table, so $A$ and $B$ need one table each: 2 tables. (The question says the relationships are "between M and N", but it describes them as running from $A$ to $B$, so $M$ and $N$ stand for $A$ and $B$.)

2. A many-to-many relationship cannot be stored as a foreign key on either side, because a row on one side relates to many rows on the other. $R2$ therefore needs its own table holding the keys of $A$ and $B$.

3. A one-to-many relationship needs no table of its own. The key of the 'one' side ($A$) is added as a foreign key to the 'many' side ($B$), so $R1$ merges into table $B$.

4. This merge is allowed even though $R2$ also links $A$ and $B$, because $R1$ and $R2$ are separate relationships with separate meanings. $B$ carries the foreign key for $R1$, and $R2$ is kept in its own table.

5. Minimum total: $A$, $B$ (with the foreign key for $R1$) and the table for $R2$, which makes $3$ tables.

6. Option (a) 4 gives $R1$ a separate table, which is not the minimum. Option (b) 2 leaves nowhere to store the many-to-many $R2$. Option (c) 6 is more than any sensible mapping needs. The answer is (d).