GATE CS 2018 Q.21 (CS Q.11): ER Model and Relational Schema Design

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 $R$ ?

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

Some entity in E1 is associated with more than one entity in E2.

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

Every entity in E2 is associated with at most one entity in E1.

Official answer and solution

Official answer: A
From: Official GATE 2018 answer key

Solution

Combine the structural constraints of many-to-one mapping and total participation on the entity set.

A relationship $R$ from entity set $E_1$ to entity set $E_2$ is many-to-one. In a many-to-one relationship from $E_1$ to $E_2$ , each entity in $E_1$ can be associated with at most one entity in $E_2$ .

Total participation of $E_1$ in $R$ means that every entity in $E_1$ must participate in $R$ , meaning it is associated with at least one entity in $E_2$ .

Combining 'at most one' and 'at least one' guarantees that every entity in $E_1$ is associated with exactly one entity in $E_2$ .

Final answer

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2018 Computer Science and Information Technology (CS)
Question: 21 of 65, printed as CS Q.11
Section: Computer Science and Information Technology

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