What defines a primary key in a Custom Data Type (CDT)?

A primary key in a Custom Data Type (CDT) is essential because it serves as a unique identifier for each row of data within that data type. This uniqueness ensures that every record can be distinctly referenced and is crucial for maintaining data integrity in databases. When a primary key is defined, it not only helps to identify individual entries but also establishes relationships with other tables or data structures, supporting the relational aspects of database design.

The requirement for uniqueness means no two rows in a data structure can have the same primary key value, which is important for data retrieval and manipulation. This makes it possible to efficiently link records and perform operations such as updates and deletions without ambiguity.

While sorting, adding additional information, or allowing multiple entries are important features of a data structure, they do not define the role of a primary key. Sorting is related to how the data is presented or organized, additional information pertains to the attributes of the data itself, and allowing multiple entries refers to cardinality, which focuses on the number of instances rather than unique identification. Thus, the defining characteristic of a primary key is its ability to uniquely identify a row of data.