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? A. Every entity in E1 is associated with exactly one entity in E2 B. Some entity in E1 is associated with more than one entity in E2 C. Every entity in E2 is associated with exactly one entity in E2 D. Every entity in E2 is associated with exactly one entity in E1

GATE 2018 · Databases · ER Diagram · medium

Answer: A. Every entity in E1 is associated with exactly one entity in E2

1. Apply many-to-one definition to E1: By definition of many-to-one from E1 to E2, every entity in E1 is associated with AT MOST one entity in E2. Since E1 also has TOTAL participation (every E1 entity must participate), 'at most one' becomes 'exactly one'. Therefore, every entity in E1 is associated with exactly one entity in E2.
2. Check option D (about E2): Option D says every E2 entity is associated with exactly one E1 entity. Since |E1| > |E2| and every E1 entity maps to some E2 entity (total), by the pigeonhole principle at least one E2 entity must be associated with 2 or more E1 entities. So D is false.
3. Confirm correct answer: Option A: 'Every entity in E1 is associated with exactly one entity in E2' — TRUE (many-to-one + total participation of E1). Option B is false (E1 entities map to AT MOST one E2 entity per many-to-one). Option C is garbled (says E2 with E2). Option D is false by pigeonhole. Answer is A.