In an Entity-Relationship (ER) model, suppose š¯‘… is a many-to-one relationshipā€¦
2018
In an Entity-Relationship (ER) model, suppose š¯‘… is a many-to-one relationship from entity set E1 to entity set E2. Assume that E1 and E2 participate totally in š¯‘… and that the cardinality of E1 is greater than the cardinality of E2.
Which one of the following is true about š¯‘…?
- 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 E1.
- D.
Every entity in E2 is associated with at most one entity in E1.
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Correct answer: A
Answer: Every entity in E1 is associated with exactly one entity in E2.
Why: A many-to-one relationship from E1 to E2 means each E1 entity maps to one and only one E2 entity. Total participation of E1 means every E1 must participate in the relationship, so every E1 has that single associated E2.
The statement that some E1 entity is associated with more than one E2 is false because that would violate the many-to-one constraint (an E1 cannot link to multiple E2).
The statement that every E2 is associated with exactly one E1 is false: in many-to-one, a single E2 can be associated with multiple E1 entities.
The statement that every E2 is associated with at most one E1 is also false for the same reason: many E1 entities may reference the same E2.
The given cardinality relation (|E1| > |E2|) is consistent with the correct statement because it implies some E2 will be linked from multiple E1 entities.
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