The correct answer is A. Has multiple candidate keys.

To understand why option A is the correct answer, let’s explore what the Third Normal Form (3NF) is and why it might be inadequate in situations where a relation has multiple candidate keys. We will also discuss why the other options are not correct:

A. Has Multiple Candidate Keys (Correct Answer):

In the context of database normalization, the Third Normal Form (3NF) is a level of normalization that a relational database schema should ideally achieve. To reach 3NF, a relation must meet the following criteria:

1. It must be in Second Normal Form (2NF), which means that it should have no partial dependencies. Partial dependencies occur when an attribute depends on only part of a candidate key, rather than the entire key.

2. It must eliminate transitive dependencies. A transitive dependency exists when a non-key attribute depends on another non-key attribute through the primary key.

Now, let’s explain why having multiple candidate keys can pose challenges to achieving or applying 3NF:

Challenges with Multiple Candidate Keys:

Complexity of Dependencies:

In a relation with multiple candidate keys, there can be complex sets of functional dependencies. These dependencies can involve multiple combinations of attributes, making it challenging to identify and eliminate all potential partial and transitive dependencies.

Determining the Key:

In such relations, it can be difficult to determine which candidate key to use as the primary key for normalization purposes. Different candidate keys may lead to different sets of dependencies, and choosing the wrong key can result in incomplete normalization.

Dependency Preservation:

Achieving 3NF while preserving all dependencies, including those based on different candidate keys, can be a complex task. Ensuring that all attributes are functionally determined by the primary key(s) is essential for normalization.

Performance and Storage Implications:

Maintaining multiple candidate keys and complex dependencies can have performance and storage implications in database systems. Queries and updates may become more complex and slower due to the presence of multiple keys and dependencies.

In summary, while 3NF is a desirable level of normalization to reduce data redundancy and improve data integrity, it can become inadequate in situations where a relation has multiple candidate keys.

The presence of multiple keys can introduce complex dependencies and challenges in determining the correct primary key for normalization, making it harder to achieve and apply 3NF effectively.

Now, let’s discuss why the other options are not correct:

B. Has Candidate Keys That Are Composite:

Having candidate keys that are composite (i.e., composed of multiple attributes) is not inherently a reason for 3NF to be inadequate. In fact, 3NF is designed to handle relations with composite candidate keys.

It requires that all non-key attributes be functionally dependent on the entire candidate key, whether it is composite or not. Therefore, 3NF can be applied effectively to relations with composite candidate keys by ensuring that all dependencies are fully satisfied.

C. Has Overlapped Candidate Keys:

Overlapped candidate keys refer to candidate keys that share some common attributes. While managing overlapped candidate keys can be more complex, it is not a reason for 3NF to be inadequate per se.

3NF can still be applied to relations with overlapped candidate keys by ensuring that all attributes are functionally determined by the candidate key(s). The presence of overlapping keys does not inherently invalidate 3NF but may require careful consideration and analysis of dependencies.

D. None of the Above:

The correct answer is not “None of the Above” because, as explained earlier, having multiple candidate keys can indeed pose challenges to achieving or applying the Third Normal Form (3NF).

The presence of multiple candidate keys can lead to complex dependencies and make it more difficult to determine the correct primary key for normalization. Therefore, option A, “Has multiple candidate keys,” is the correct answer as it accurately identifies a situation where 3NF may be inadequate due to the associated complexities.