What describes a Perfect Key in a data model?

A Perfect Key in a data model refers to a situation where every row in a table has a unique identifier that distinctly identifies each record. This concept is critical for establishing relationships between tables and ensuring that each row can be retrieved without ambiguity.

The reason the selected answer is correct is that it emphasizes the necessity for uniqueness in the key values across the rows. When every row contains a unique key value, it ensures that there are no duplicates. This uniqueness allows for efficient data retrieval, prevents data redundancy, and ensures the integrity of relationships within the data model.

In contrast, other options reflect characteristics that would undermine the function of a Perfect Key. For instance, if key values were allowed to be duplicate, as stated in one of the other choices, it would complicate data relationships and potentially lead to confusion when defining associations in the data model. The presence of non-unique or missing key values would also negate the concept of a Perfect Key, as it would not guarantee that each row can be uniquely identified and linked reliably to other tables in the model.

Thus, the defining trait of a Perfect Key is that every row has a unique key value, clearly capturing the essence of what constitutes a well-structured data model.