Which of the following is NOT a characteristic of a primary key?

A primary key is an essential component of a database that ensures each record within a table can be uniquely identified. The characteristic of supporting NULL values is not applicable to primary keys, as primary keys must always contain valid data for every record. This means that every value in a primary key column must be unique and cannot be NULL, ensuring complete identification of each record without ambiguity.

The characteristics that define a primary key include:

• Uniqueness: Each record must have a unique key value, ensuring no duplicate records exist in that column.
• Stability: A primary key should ideally not change over time, ensuring consistent identification of records.
• Identification: The primary key serves the fundamental purpose of identifying a record in a table, enabling efficient data retrieval and relationships between tables.

Thus, supporting NULL values contradicts the fundamental requirements of a primary key, reinforcing why this is the correct answer.