GATE CS 2018 Set 1 — Question 21
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Databases → ER Model → Cardinality & Participation
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Question
In an Entity-Relationship (ER) model, suppose is a many-to-one relationship from entity set E1 to entity set E2. Assume that E1 and E2 participate totally in and that the cardinality of E1 is greater than the cardinality of E2.Which one of the following is true about ?
Correct answer
(A) Every entity in E1 is associated with exactly one entity in E2.
Solution
1.Many-to-one relationship from E1 to E2: This means that each entity in E1 can be associated with at most one entity in E2, while an entity in E2 can be associated with multiple entities in E1.
2.Total participation of E1 in : This means every entity in E1 must be associated with at least one entity in E2.
3.Combining these two facts: Since every entity in E1 is associated with at least one entity in E2 (total participation) and at most one entity in E2 (many-to-one constraint), it follows that every entity in E1 is associated with exactly one entity in E2.
4.Since and there is total participation, by the Pigeonhole Principle, at least one entity in E2 must be associated with more than one entity in E1, making options (C) and (D) false.
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