A fan trap in the ER model

A fan trap is a situation that can occur in an Entity-Relationship (ER) model, where inconsistencies arise due to the way relationships between entities are defined. It can cause ambiguity and difficulties in interpreting the relationships and querying the database.

Explanation of the answer

The correct answer to the question is option B, which states that a fan trap may exist in the ER model if two or more one-to-many relationships fan out from the same entity. Let's understand why this is the case.

One-to-many relationships

In an ER model, a one-to-many relationship represents the relationship between two entities where one entity can have multiple instances associated with it in the other entity. For example, consider an ER model for a university where the entities are "Department" and "Professor". A one-to-many relationship can exist between these entities, where one department can have multiple professors.

Fan trap

A fan trap occurs when there are overlapping one-to-many relationships between entities, leading to a situation where it is difficult to interpret the relationships accurately. This can create inconsistencies and confusion in the model.

Illustration of a fan trap scenario

Let's consider an example to illustrate a fan trap scenario:

Entities:
- Student
- Course
- Department

Relationships:
1. Student takes Course (one-to-many)
2. Department offers Course (one-to-many)
3. Student belongs to Department (one-to-one)

In this scenario, there is a fan trap because there are two one-to-many relationships (1 and 2) that fan out from the same entity (Course) and converge at the same entity (Department).

Consequences of a fan trap

A fan trap can lead to ambiguity and difficulties in interpreting the relationships and querying the database. For example, if we want to retrieve the students belonging to a specific department, we may encounter ambiguous results because of the overlapping relationships.

Resolution of a fan trap

To resolve a fan trap, a new relationship called a "fan trap resolver" can be introduced. In the above example, a new relationship could be created between the Student and Department entities to resolve the fan trap.

Conclusion

In summary, a fan trap occurs in the ER model when there are overlapping one-to-many relationships that fan out from the same entity. This can lead to ambiguity and difficulties in interpreting the relationships and querying the database. Resolving a fan trap may involve introducing additional relationships to clarify the model.